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https://github.com/next-multilingual/next-multilingual

An opinionated end-to-end solution for Next.js applications that requires multiple languages.
https://github.com/next-multilingual/next-multilingual

i18n multilingual nextjs reactjs

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An opinionated end-to-end solution for Next.js applications that requires multiple languages.

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README 

        
# ![next-multilingual](./assets/next-multilingual-banner.svg)



`next-multilingual` is an opinionated end-to-end solution for Next.js applications **✱** that requires multiple languages.



Check out our [demo app](https://next-multilingual-example.vercel.app)!



[![Try me](https://img.shields.io/badge/-Try%20me!-green?style=for-the-badge)](https://next-multilingual-example.vercel.app)



**✱** `next-multilingual` only works with πŸ“ `pages` directory. We are still ironing out our solution to support the new πŸ“ `app` directory since [internationalization](https://nextjs.org/docs/app/building-your-application/routing/internationalization) is no longer part of Next.js' config.



## Installation πŸ’»



```

npm install next-multilingual

```



## What's in it for me? πŸ€”



- The enforcement of i18n best practices across your entire application.

- Modular messages (also known as "localized strings") that work just like CSS modules (no more monolithic files).

- A powerful `useMessages` hook that supports [ICU MessageFormat](https://unicode-org.github.io/icu/userguide/format_parse/messages/) and JSX injection out of the box.

- The ability to use localized URLs (e.g., `/en-us/contact-us` for U.S. English and `/fr-ca/nous-joindre` for Canadian French) **✱**.

- All page URLs will use locale prefixes (related to this [discussion](https://github.com/vercel/next.js/discussions/18419)).

- Can easily be configured with smart locale detection that dynamically renders the homepage, without using redirections.

- Automatically generate canonical and alternate links optimized for SEO.



**✱** Your default locale slugs must match the `pages` directory file system (e.g., a slug for "About us" should be in an `about-us` directory). If the default locale you require uses characters beyond those supported by the file system, it hasn't been tested and will likely not work. Pull requests are welcome 😊.



## Before we start πŸ’Ž



`next-multilingual` has put a lot of effort into adding [TSDoc](https://tsdoc.org/) to all its APIs. Please check directly in your IDE if you are unsure how to use certain APIs provided in our examples.



Also, having an opinion on "best practices" is not an easy task. This is why we documented our design decisions in a special document that can be consulted [here](./docs/design-decisions.md). If you feel that some of our APIs don't offer what you would expect, make sure to consult this document before opening an issue.



## Getting Started πŸ’¨



For those who prefer to jump right into the action, look in the [`example`](./example) directory for an end-to-end implementation of `next-multilingual`. For the rest, the section below will provide a complete, step by step, configuration guide.



## Step by step configuration βš™οΈ



### Configure Next.js



There are many options to configure in Next.js to achieve our goals. `next-multilingual` mostly cares about:



- Your unique application identifier: this will be used to ensure that your messages (localized strings) have unique identifiers.

- Your locales: we only support BCP47 language tags that contain both a country and language code.



We offer two APIs to simplify this step:



#### 〰️ `getConfig` (simple config)



This function will generate a Next.js config that will meet most use cases. `getConfig` takes the following arguments:



- `applicationId` β€” The unique application identifier that will be used as a messages key prefix.

- `locales` β€” The locales of your application.

- `defaultLocale` - The default locale of your application (it must also be included in `locales`)

  > ❗ Only BCP 47 language tags following the `language`-`country` format are accepted. For more details on why, refer to the [design decisions](./docs/design-decisions.md) document.

- `options` (optional) β€” Options part of a [Next.js configuration](https://nextjs.org/docs/api-reference/next.config.js/introduction) object.



`getConfig` will return a [Next.js configuration](https://nextjs.org/docs/api-reference/next.config.js/introduction) object.



To use it, simply add the following code in your application's `next.config.js`:



```js

const { getConfig } = require('next-multilingual/config')



const config = getConfig('exampleApp', ['en-US', 'fr-CA'], 'en-US', {

  // Put your optional options below.

  poweredByHeader: false,

})



module.exports = config

```



Not all configuration options are not supported by `getConfig`. If you ever happen to use one, an error message will point you directly to the next section: advanced config.



#### 〰️ `Config` (advanced config)



If you have more advanced needs, you can use the `Config` object directly and insert the configuration required by `next-multilingual` directly in an existing `next.config.js`. The arguments of `Config` are almost identical to `getConfig` (minus the `options`) - check in your IDE (TSDoc) for details. Here is an example of how it can be used:



```js

const { Config, webpackConfigurationHandler } = require('next-multilingual/config')



const config = new Config('exampleApp', ['en-US', 'fr-CA'], 'en-US')



module.exports = {

  reactStrictMode: true,

  i18n: {

    locales: config.getUrlLocalePrefixes(),

    defaultLocale: config.getDefaultUrlLocalePrefix(),

    localeDetection: false,

  },

  poweredByHeader: false,

  webpack: webpackConfigurationHandler,

}

```



If you need to customize your own Webpack configuration, we recommend extending our handler like this:



```ts

import Webpack from 'webpack'



import { webpackConfigurationHandler, WebpackContext } from 'next-multilingual/config'



export const myWebpackConfigurationHandler = (

  config: Webpack.Configuration,

  context: WebpackContext

): Webpack.Configuration => {

  const myConfig = webpackConfigurationHandler(config, context)

  // Do stuff here.

  return myConfig

}

```



Or directly in `next.config.js`:



```js

// Webpack handler wrapping next-multilingual's handler.

const webpack = (config, context) => {

  config = webpackConfigurationHandler(config, context)

  // Do stuff here.

  return config

}

```



#### How does it work?



`next-multilingual/config` does 2 things leveraging Next.js' current routing capability:



1. Add [Rewrites](https://nextjs.org/docs/api-reference/next.config.js/rewrites) to link localized URLs to the default language URLs.

2. Add [Redirects](https://nextjs.org/docs/api-reference/next.config.js/redirects) to redirect all possible encoded URL [forms](https://unicode.org/reports/tr15/) to the normalized [NFC](https://en.wikipedia.org/wiki/Unicode_equivalence) URL.



`next-multilingual/config` also handles the special Webpack configuration required for server side rendering of localized

URLs using `next-multilingual/link/ssr` for `Link` components and `next-multilingual/head/ssr` for canonical and alternate links in the `Head` component.



For more details on the implementation such as why we are using UTF-8 characters, refer to the [design decisions](./docs/design-decisions.md) document.



### Configure our Babel plugin



#### 〰️ `next-multilingual/messages/babel-plugin`



To display localized messages with the `useMessages()` hook, we need to configure our custom [Babel](https://babeljs.io/) plugin that will automatically inject strings into pages and components. The [recommended way](https://nextjs.org/docs/advanced-features/customizing-babel-config) to do this is to include a `.babelrc` at the base of your application:



```json

{

  "presets": ["next/babel"],

  "plugins": ["next-multilingual/messages/babel-plugin"]

}

```



If you do not configure the plugin you will get an error when trying to use `useMessages`.



### Create a custom `App` (`_app.tsx`)



We need to create a [custom `App`](https://nextjs.org/docs/advanced-features/custom-app) by adding [`_app.tsx`](./example/pages/_app.tsx) in the `pages` directory:



```ts

import { useActualLocale } from 'next-multilingual'

import type { AppProps } from 'next/app'



const ExampleApp: React.FC = ({ Component, pageProps }) => {

  useActualLocale() // Forces Next.js to use the actual (proper) locale.

  return 

}



export default ExampleApp

```



This basically does two things, as mentioned in the comments:



1. Inject the actual locale in Next.js' router since we need to use a "fake default locale".

2. By default, persist the actual locale in the cookie so we can reuse it when hitting the homepage without a locale (`/`). If you do not want to use `next-multilingual`'s locale detection you can use `useActualLocale(false)` instead.



### Create a custom `Document` (`_document.tsx`)



We also need to create a [custom `Document`](https://nextjs.org/docs/advanced-features/custom-document) by adding [`_document.tsx`](./example/pages/_document.tsx) in the `pages` directory:



```ts

import { getHtmlLang } from 'next-multilingual'

import { DocumentProps, Head, Html, Main, NextScript } from 'next/document'



const Document: React.FC = (documentProps) => {

  return (

    

      

        

      

      

        

        

      

    

  )

}



export default Document

```



This serves only 1 purpose: display the correct server side locale in the `` tag. Since we are using a "fake" default locale, it's important to keep the correct SSR markup, especially when resolving a dynamic locale on `/`.



### Configure all your pages to use SEO friendly markup



`next-multilingual/head` provides a `` component which automatically creates a canonical link and alternate links in the header. This is something that is not provided out of the box by Next.js.



#### Add a `NEXT_PUBLIC_ORIGIN` environment variable



As per [Google](https://developers.google.com/search/docs/advanced/crawling/localized-versions), alternate links must be fully-qualified, including the transport method (http/https). Because Next.js does not know which URL is used at build time, we need to specify the absolute URL that will be used, in an [environment variable](https://nextjs.org/docs/basic-features/environment-variables). For example, for the development environment, create an `.env.development` file at the root of your application with the following variable (adjust based on your setup):



```ini

NEXT_PUBLIC_ORIGIN=http://localhost:3000

```



Regardless of the environment, `next-multilingual` will look for a variable called `NEXT_PUBLIC_ORIGIN` to generate fully-qualified URLs. If you are using Next.js' [`basePath`](https://nextjs.org/docs/api-reference/next.config.js/basepath), it will be added automatically to the base URL.



`NEXT_PUBLIC_ORIGIN` will only accept fully qualified domains (e.g., `http://example.com`), without any paths.



## Using `next-multilingual` 🎬



Now that everything has been configured, we can focus on using `next-multilingual`!



### Getting the correct locales values



In order to get `next-multilingual` to work as designed, we had to find solutions to 2 problems:



1. `undefined` values: because Next.js support sites without locales, it's native types allows to have `undefined` values, which for our case is more of an annoyance and requires extra casting.

2. wrong values: because [Next.js does not allow to have locale prefix for the default locale](https://github.com/vercel/next.js/discussions/18419), `next-multilingual` has to create a default locale that we never use. This means that to access the relevant locale information, we cannot rely on Next.js' APIs.



We created the following APIs to allow consistent locales values across your application:



#### `useRouter`



This is a simple wrapper on top of Next.js' `useRouter` which both provides the correct locales but also never returns `undefined`.



```ts

import { NextPage } from 'next'

import { useRouter } from 'next-multilingual/router'



const Page: NextPage = () => {

  const router = useRouter()

  return <>{router.locale}>

}



export default Page

```



#### `getStaticPropsLocales`



```ts

import { getStaticPropsLocales } from 'next-multilingual'



export const getStaticProps: GetStaticProps = async (context) => {

  const { locale, locales, defaultLocale } = getStaticPropsLocales(context)

  // do stuff

  return { props: {} }

}

```



#### `getStaticPathsLocales`



```ts

import { getStaticPathsLocales } from 'next-multilingual'



export const getStaticPaths: GetStaticProps = async (context) => {

  const { locales, defaultLocale } = getStaticPathsLocales(context)

  // do stuff

  return { props: {} }

}

```



#### `getServerSidePropsLocales`



```ts

import { getServerSidePropsLocales } from 'next-multilingual'



export const getServerSideProps: GetServerSideProps = async (context) => {

  const { locale, locales, defaultLocale } = getServerSidePropsLocales(context)

  // do stuff

  return { props: {} }

}

```



### Creating the homepage



> ⚠️ Note that while we recommend using smart locale detection to dynamically render the homepage, this is completely optional. By using advanced configuration with `localeDetection: true`, you will restore the default Next.js behavior without the need of using `getServerSideProps`.



The homepage is a bit more complex than other pages, because we need to implement dynamic locale detection (and display) for the following reason:



- Redirecting on `/` can have a negative impact on SEO and is not the best user experience.

- `next-multilingual` comes with a `getPreferredLocale` API that offers smarter auto-detection than the default Next.js implementation.



You can find a full implementation in the [example](./example/pages/index.tsx), but here is a stripped down version:



```tsx

import type { GetServerSideProps, NextPage } from 'next'

import { ResolvedLocaleServerSideProps, resolveLocale, useResolvedLocale } from 'next-multilingual'

import { useRouter } from 'next-multilingual/router'



const Homepage: NextPage = ({ resolvedLocale }) => {

  // Force Next.js to use a locale that was resolved dynamically on the homepage (this must be the first action on the homepage).

  useResolvedLocale(resolvedLocale)

  const { locale } = useRouter()



  return 
{locale}


}



export default Homepage



export const getServerSideProps: GetServerSideProps = async (

  context

) => {

  return {

    props: {

      resolvedLocale: resolveLocale(context),

    },

  }

}

```



In a nutshell, this is what is happening:



1. Let the server get the best locale for the page by:

   - Checking if a previously used locale is available in the `next-multilingual`'s locale cookie.

   - Otherwise, use smart locale detection based on the user's browsers settings.

2. The server then passes the resolved locale back to the client and:

   - The client overwrites the value on the router using `useResolvedLocale` to make this dynamic across the application.

   - The value is also stored back in the cookie to keep the selection consistent



### Creating messages



Every time that you create a `tsx`, `ts`, `jsx` or `js` (compilable) file and that you need localized messages, you can simply create a message file in your supported locales that will only be usable by these files. Just like CSS modules, the idea is that you can have message files associated with another file's local scope. This has the benefit of making messages more modular and also avoids sharing messages across different contexts (more details in the [design decisions document](./docs/design-decisions.md) on why this is bad).



Message files have 2 main use cases:



- **Localized URLs**: for the pages in your `pages` directory, you can specify a localized URL segment (part of a URL in between `/` or at the end of the path) using the `slug` key identifier. More details on how to do this below.

- **All localizable strings**: they will store all the localizable strings (messages) used by your application. Each compilable file can have their own messages. Those messages will be available in local scope only, using the `useMessages` hook. Imagine CSS but for localizable strings.



To summarize:



- Messages are associated to a compilable file and should only be used in that local scope.

- Messages are used both to localize URLs and to display localized text everywhere in your application.

- You should only use this method in your application to simplify your localization process.



#### How do these files work?



Creating and managing those files is as simple as creating a style sheet, but here are the important details:



- The message files are `.properties` files. Yes, you might wonder why, but there are good reasons documented in the [design decision document](./docs/design-decisions.md).

- Make sure your file encoding is set to `UTF-8`. Not doing so will replace non-Latin characters with `οΏ½`.

- To leverage some of the built-in IDE support for `.properties` files, we follow a strict naming convention: `..properties`

- Each message must have unique keys that follow a strict naming convention: `..` where:

  - **applicationId** must use the same value as set in `next-multilingual/config`

  - **context** must represent the context associated with the message file, for example `aboutUsPage` or `footerComponent` could be good examples of context. Each file can only contain 1 context and context should not be used across many files as this could cause "key collision" (non-unique keys).

  - **id** is the unique identifier in a given context (or message file).

  - Each "segment" of a key must be separated by a `.` and can only contain between 1 to 50 alphanumeric characters - we recommend using camel case for readability.

- For pages:

  - If you want to localize your URLs, you must include message files that include a key with the `slug` identifier.

  - If you want to customize your title with a description longer than the slug, include a key with the `title` identifier.

  - Use the `getTitle` API provided in `next-multilingual/messages` to automatically fallback between the `title` and `slug` keys.

- For components, files are only required if you use the `useMessages` hook.

- For messages shared across multiple components (shared messages), you need to create a "shared message hook". More details on how to do this below.



Also, make sure to check your console log for warnings about potential issues with your messages. It can be tricky to get used to how it works first, but we tried to make it easy to detect and fix problems. Note that those logs will only show in non-production environments.



#### Using messages outside of hooks



It's not uncommon to need localized messages while being unable to use hooks. An example would be while using one of Next.js' core features is its [builtin API support](https://nextjs.org/docs/api-routes/introduction). In that context, instead of using `useMessage` we can simply use `getMessages` while specifying the `locale` argument.



#### Using messages for localized URLs



As mentioned previously, there is one special key for `pages`, where the `id` is `slug`. Unlike traditional slugs that look like `this-is-a-page`, we ask you to write the slug as a normal and human readable sentence, so that it can be translated like any other string. This avoids having special processes for slugs which can be costly and complex to manage in multiple languages.



Basically, the `slug` is the human readable "short description" of your page, and represents a segment (part between `/` or at the end of the path) of a URL. When used as a URL segment, the following transformation is applied:



- all characters will be lowercased

- any character that is not a letter or number in all languages will be replaced by `-`



For example, `About Us` will become `about-us`.



For the homepage, the URL will always be `/` which means that `slug` keys will not be used to create localized URL segments.



Don't forget, slugs must be written as a normal short description, which means skipping words to keep it shorter for SEO is discouraged. The main reason for this, is that if you write "a bunch of keywords", a linguist who is not familiar with SEO might have a hard time translating that message. Having SEO specialists in many languages would also be very costly and difficult to scale. In an ideal scenario, market-specific SEO pages should probably be authored and optimized in the native languages, but this is no longer part of the translation process. `next-multilingual`'s focus is to provide an easy, streamlined solution to localize URLs in many languages.



The `slug` key will also be used as a fallback of the `title` key when using the `getTitle` API provided in `next-multilingual/messages`. This API makes it easy to customize titles when a slug feels insufficient.



> ⚠️ Note that changing a `slug` value means that a URL will change. Since those changes are happening in `next.config.js`, like any Next.js config change, the server must be restarted to see the changes in effect. The same applies if you change the folder structure since the underlying configuration relies on this.



If you want to have a directory without any pages, you can still localize it by creating an `index..properties` file (where `locale` are the locales you support). While this option is supported, we don't recommend using it as this will make URL paths longer which goes against SEO best practice.



By default, `next-multilingual` will exclude some files like custom error pages, or any [API routes](https://nextjs.org/docs/api-routes/introduction) under the `/api` directory. You can always use `slug` keys when using messages for these files, but they will not be used to create localized URLs.



#### What do message files look like?



You can always look into the [example](./example) to see message files in action, but here is a sample that could be used on the homepage:



```properties

# Homepage title

exampleApp.homepage.title = Homepage

# Homepage headline

exampleApp.homepage.headline = Welcome to the homepage

```



### Creating other pages



Now that we learned how to create the homepage and some of the details around how things work, we can easily create other pages. We create many pages in the [example](./example), but here is a sample of what `about-us.jsx` could look like:



```jsx

import { NextPage } from 'next'

import { getTitle, useMessages } from 'next-multilingual/messages'



import Layout from '@/layout'



import styles from './index.module.css'



const AboutUs: NextPage = () => {

  const messages = useMessages()

  const title = getTitle(messages)

  return (

    

      
{title}


      
{messages.format('details')}


    

  )

}



export default AboutUs

```



And of course you would have this message file `about-us.en-US.properties`:



```properties

# Page localized URL segment (slug) in (translatable) human readable format.

# This key will be "slugified" (e.g, "About Us" will become "about-us"). All non-alphanumeric characters will be replaced by "-".

exampleApp.aboutUsPage.slug = About Us

# Page details.

exampleApp.aboutUsPage.details = This is just some english boilerplate text.

```



### Adding links



`next-multilingual` comes with its own `` component that allows for client side and server side rendering of localized URL. It's usage is simple, it works exactly like Next.js' [``](https://nextjs.org/docs/api-reference/next/link).



The only important thing to remember is that the `href` attribute should always contain the Next.js URL. Meaning, the file structure under the `pages` folder should be what is used and not the localized versions.



In other words, the file structure is considered as the "non-localized" URL representation, and `` will take care of replacing the URLs with the localized versions (from the messages files), if they differ from the structure.



The API is available under `next-multilingual/link` and you can use it like this:



```tsx

import Link from 'next-multilingual/link'

import { useMessages } from 'next-multilingual/messages'



const Menu: React.FC = () => {

  const messages = useMessages()

  return (

    

      

        {messages.format('home')}

      

      

        {messages.format('aboutUs')}

      

      

        {messages.format('contactUs')}

      

    

  )

}



export default Menu

```



Each of these links will be automatically localized when the `slug` key is specified in that page's message file. For example, in U.S. English the "Contact Us" URL path will be `/en-us/contact-us` while in Canadian French it will be `/fr-ca/nous-joindre`.



#### What about server side rendering?



As the data for this mapping is not immediately available during rendering, `next-multilingual/link/ssr` will take care of the server side rendering (SSR). By using `next-multilingual/config`'s `getConfig`, the Webpack configuration will be added automatically. If you are using the advanced `Config` method, this explains why the special Webpack configuration is required in the example provided prior.



### Using localized URLs in other components



Not all localized URLs are using the `` component and this is also why Next.js has the `router.push` method that can be used by many other use cases. `next-multilingual` can support these use cases with the `useLocalizedUrl` hook that will return a localized URL, usable by any components. Here is an example on how it can be leveraged:



```tsx

import { NextPage } from 'next'

import { useMessages } from 'next-multilingual/messages'

import { useLocalizedUrl } from 'next-multilingual/url'

import router from 'next/router'



const Tests: NextPage = () => {

  const messages = useMessages()

  const localizedUrl = useLocalizedUrl('/about-us')

  return  router.push(localizedUrl)}>{messages.format('clickMe')}

}



export default Tests

```



If you prefer to define your URLs inline instead than at the top of the component, or if you need to do more advanced URL manipulations, you can also use the `useGetLocalizedUrl` hook that returns a function to get URLs:



```tsx

import { NextPage } from 'next'

import { useMessages } from 'next-multilingual/messages'

import { useGetLocalizedUrl } from 'next-multilingual/url'

import router from 'next/router'



const Tests: NextPage = () => {

  const messages = useMessages()

  const { getLocalizedUrl } = useGetLocalizedUrl()

  return (

     router.push(getLocalizedUrl('/about-us'))}>

      {messages.format('clickMe')}

    

  )

}



export default Tests

```



> ⚠ Be careful, if you want to use the string value of the URL inside React elements, you will have errors because the URLs differ between pre-rendering and the browser. The reason for this is that on the client, on first render, Next.js doesn't have access to the rewrites data, and therefore uses "semi-localized" URL paths (e.g. `/fr-ca/about-us`). Since this is a native Next.js behavior, the simplest way to work around this for now is by adding `suppressHydrationWarning={true}` to your element. To work around this, `useGetLocalizedUrl` also returns a `isLoading` property that can be used to track when the localized URLs are available to use on the client.



### Getting localized URLs without a hook



You might run into situation where you also need to get a localized URL but using a hook is not an option. This is where `getLocalizedUrl` in `next-multilingual/url` comes in. It acts the same as `useLocalizedUrl` but its `locale` argument is mandatory.



Imagine using Next.js' API to send transactional emails and wanting to leverage `next-multilingual`'s localized URLs without having to hardcode them in a configuration. Here is an example of how it can be used:



```ts

import type { NextApiRequest, NextApiResponse } from 'next'

import { isLocale } from 'next-multilingual'

import { getMessages } from 'next-multilingual/messages'

import { getLocalizedUrl } from 'next-multilingual/url'

import { sendEmail } from 'send-email'



/**

 * The "/api/send-email" handler.

 */

const handler = (request: NextApiRequest, response: NextApiResponse): Promise => {

  const locale = request.headers['accept-language']

  let emailAddress = ''



  try {

    emailAddress = JSON.parse(request.body).emailAddress

  } catch (error) {

    response.status(400)

    return

  }



  if (locale === undefined || !isLocale(locale) || !emailAddress.length) {

    response.status(400)

    return

  }



  const messages = getMessages(locale)

  sendEmail(

    emailAddress,

    messages.format('welcome', { loginUrl: await getLocalizedUrl('/login', locale, true) })

  )

  response.status(200)

}



export default handler

```



### Creating components



Creating components is the same as pages but they live outside the `pages` directory. Also, the `slug` key (if used) will not have any impact on URLs. We have a few [example components](./example/components) that should be self-explanatory but here is an example of a `Footer.tsx` component:



```tsx

import { useMessages } from 'next-multilingual/messages'



const Footer: React.FC = () => {

  const messages = useMessages()

  return {messages.format('footerMessage')}

}



export default Footer

```



And its messages file:



```properties

# This is the message in the footer at the bottom of pages

exampleApp.footerComponent.footerMessage = Β© Footer

```



Also make sure to look at the [language switcher component example](./example/components/LanguageSwitcher.tsx) that is a must in all multilingual applications.



### Creating shared messages



We've been clear that sharing messages is a bad practice from the beginning, so what are we talking about here? In fact, sharing messages by itself is not bad. What can cause problems is when you share messages in different contexts. For example, you might be tempted to create a `Button.ts` shared message file containing `yesButton`, `noButton` keys - but this would be wrong. In many languages simple words such as "yes" and "no" can have different spellings depending on the context, even if it's a button.



When is it good to share messages? For lists of items.



For example, to keep your localization process simple, you want to avoid storing localizable strings in your database (more details on why in the [design decision document](./docs/design-decisions.md)). In your database you would identify the context using unique identifiers and you would store your messages in shared message files, where your key's identifiers would match the ones from the database.



To illustrate this we created [one example using fruits](./example/src/messages/fruits/useFruitsMessages.ts). All you need to do, is create a hook that calls `useMessages` like this:



```ts

export { useMessages as useFruitsMessages } from 'next-multilingual/messages'

```



> ❗ If you need to access your messages outside of hooks, you need to also export `getMessages`.



Of course, you will have your messages files in the same directory:



```properties

exampleApp.fruits.banana = Banana

exampleApp.fruits.apple = Apple

exampleApp.fruits.strawberry = Strawberry

exampleApp.fruits.grape = Grape

exampleApp.fruits.orange = Orange

exampleApp.fruits.watermelon = Watermelon

exampleApp.fruits.blueberry = Blueberry

exampleApp.fruits.lemon = Lemon

```



And to use it, simple import this hook from anywhere you might need these values:



```tsx

import { useFruitsMessages } from '../messages/useFruitsMessages'



const FruitList: React.FC () => {

  const fruitsMessages = useFruitsMessages()

  return (

    <>

      {fruitsMessages

        .getAll()

        .map((message) => message.format())

        .join(', ')}

    >

  )

}



export default FruitList

```



You can also call individual messages like this:



```ts

fruitsMessages.format('banana')

```



The idea to share those lists of items is that you can have a consistent experience across different components. Imagine a dropdown with a list of fruits in one page, and in another page an auto-complete input. But the important part to remember is that the list must always be used in the same context, not to re-use some of the messages in a different context.



### Message Placeholders



Using placeholders in messages is a critical functionality as not all messages contain static text. `next-multilingual` supports the [ICU MessageFormat](https://unicode-org.github.io/icu/userguide/format_parse/messages/) syntax out of the box which means that you can use the following message:



```properties

exampleApp.homepage.welcome = Hello {name}!

```



And inject back the values using:



```ts

messages.format('welcome', { name: 'John Doe' })

```



#### How to use `format`



There are a few simple rules to keep in mind when using `format`:



- If you do not provide the `values` argument when formatting the message, it will simply output the message as static text.

- If you provide the `values` argument when formatting the message, you must include the values of all placeholders using the `{placeholder}` syntax in your message. Otherwise the message will not be displayed.

- If you provide `values` that are not in your message, they will be silently ignored.



#### Plurals



One of the main benefits of ICU MessageFormat is to use Unicode's tools and standards to enable applications to sound fluent in most languages. A lot of engineers might believe that by having 2 messages, one for singular and one for plural is enough to stay fluent in all languages. In fact, Unicode documented the [plural rules](https://unicode-org.github.io/cldr-staging/charts/latest/supplemental/language_plural_rules.html) of over 200 languages and some languages like Arabic can have up to 6 plural forms.



To ensure that your sentence will stay fluent in all languages, you can use the following message:



```properties

exampleApp.homepage.mfPlural = {count, plural, =0 {No candy left.} one {Got # candy left.} other {Got # candies left.}}

```



And the correct plural form will be picked, using the correct plural categories defined by Unicode:



```ts

messages.format('mfPlural', { count })

```



There is a lot to learn on this topic. Make sure to read the Unicode documentation and [try the syntax yourself](https://format-message.github.io/icu-message-format-for-translators/editor.html) to get more familiar with this under-hyped i18n capability.



#### Escaping Curly Brackets



In a rare event where you would need to use both placeholders using the `{placeholder}` syntax and also display the `{` and `}` characters in a message, you will need to replace them by the `{` (for `{`) and `}` (for `}`) HTML entities which are recognized by translation tools like this:



```properties

exampleApp.debuggingPage.variableInfo = Your variable contains the following values: {{values}}

```



If you have a message without values (placeholders), escaping `{` and `}` with HTML entities is not required and will display entities as static text.



#### Injecting JSX



It is a very common situation that we need to have inline HTML, inside a single message. One way to do this would be:



```properties

# Bad example, do not ever do this!

exampleApp.homepage.createAccount1 = Please

exampleApp.homepage.createAccount2 = create your account

exampleApp.homepage.createAccount3 = today for free.

```



And then:



```jsx



  {messages.format('createAccount1')}

  {messages.format('createAccount2')}

  {messages.format('createAccount3')}



```



There are 2 problems with this approach:



1. Translating a broken sentence like this can create quality issues since its not always obvious what the full sentence looks like in the tools linguists use.

2. If you need to support right-to-left language like Hebrew and Arabic, this will no longer work because the sentence's order is hardcoded in the JSX.



This is actually an anti-pattern called _concatenation_ and should always be avoided. This is the correct way to do this, using `formatJsx`:



```properties

exampleApp.homepage.createAccount = Please create your account today for free.

```



And then:



```jsx

{messages.formatJsx('createAccount', { link:  })}


```



#### How to use `formatJsx`



`formatJsx` support both placeholders and JSX elements as `values` which means that you can benefit from the standard `format` features (e.g., plurals) while injecting JSX elements.



There are a few simple rules to keep in mind when using `format`:



1. The inline _XML_ in your message is not HTML or JSX - it is merely a way to identify where to put your JSX element while enforcing opening and closing tags.

2. XML is used so that translation tools are able to keep the correct open/close order in a sentence.

3. The inline XML does not support any attributes - your attributes should be in the JSX elements you are passing as an argument to `formatJsx`.

4. The name of your XML tag will be the name you need to use when passing the JSX element in argument. For example, for a `` XML tag, the JSX element needs to be provided using `link: `.

5. XML tag names must be unique in a message and cannot be repeated. This means that even if you use `` many times in a sentence, you will need to create unique tags like ``, ``, etc. and pass their values in argument as JSX elements.

6. Using the same name for a placeholder and XML tag name (e.g., `Hello {name}`) is not supported.

7. JSX elements passed in arguments must always be closed, otherwise they are not valid JSX.

8. JSX elements passed in arguments cannot contain messages. The messages must always be in the `.properties` file.

9. JSX elements passed in arguments can contain children but each child must be unique. For example `` is valid but `
` is invalid. Instead you must use same-level XML markup in the `.properties` file and not as a JSX argument.



#### Escaping `<` and `>`



When using `formatJsx` you will still need to [escape curly brackets](#user-content-escaping-curly-brackets) if you want to display them as text. Additionally, since we will be using XML in the `formatJsx` messages, similar rules will apply to `<` and `>` which are used to identify tags.



In a rare event where you would need to inject JSX in a message using the `` (XML) syntax and also display the `<` and `>` characters in a message, you will need to replace them by the `<` (for `<`) and `>` (for `>`) HTML entities which are recognized by translation tools like this:



```properties

exampleApp.statsPage.targetAchieved = You achieved your weekly target (<5) and are eligible for a reward.

```



### Anchor Links



[Anchor links](https://www.macmillandictionary.com/dictionary/british/anchor-link) are links that takes you to a particular place in a document rather than the top of it.



One of `next-multilingual`'s core feature is supporting localized URLs. Our design has been built using normal sentences that are easy to localize and then transformed into SEO-friendly slugs. We can use the same function to slugify anchor links, so that instead of having `/fr-ca/nous-joindre#our-team` you can have `/fr-ca/nous-joindre#notre-Γ©quipe`.



There are two type of anchor links:



#### Anchor Links Used on the same page



If the anchor links are on the same page, and not referred on any other pages, you can simply add them in the `.properties` file associate with that page like this:



```properties

# Table of content header

exampleApp.longPage.tableOfContent = Table of Content



# This key will be used both as content and "slugified". Make sure when translating that its value is unique.

exampleApp.longPage.p1Header = Paragraph 1

# "Lorem ipsum" text to make the (long) page scroll

exampleApp.longPage.p1 = Lorem ipsum dolor sit amet...

```



And then the page can use the `slugify` function to link to to the unique identifier associated with the element you want to point the URL fragment to:



```jsx

import { NextPage } from 'next'

import Link from 'next-multilingual/link'

import { slugify, useMessages } from 'next-multilingual/messages'

import { useRouter } from 'next/router'



const LongPage: NextPage = () => {

  const messages = useMessages()

  const { locale } = useRouter()



  return (

    


      


        
{messages.format('tableOfContent')}


        
    

              
  • 

                

                  {messages.format('p1Header')}

                

              
    • 

            


      


      


        
{messages.format('p1Header')}


        
{messages.format('p1')}


      


    


  )

}



export default LongPage

```



#### Anchor Links used across pages



It's also common to use anchor links across pages, so that when you click a link, your browser will directly show the relevant content on that page. To do this, you need to make your page's message available to other pages by adding this simple export that will act just like "shared messages":



```tsx

export const useLongPageMessages = useMessages

```



And then you can use this hook from another page like this:



```tsx

import { NextPage } from 'next'

import Link from 'next-multilingual/link'

import { slugify, useMessages } from 'next-multilingual/messages'

import { useRouter } from 'next/router'



import { useLongPageMessages } from './long-page'



const AnchorLinks: NextPage = () => {

  const messages = useMessages()

  const { locale, pathname } = useRouter()

  const longPageMessages = useLongPageMessages()



  return (

    


      


        

          {messages.format('linkAction')}

        

      


    


  )

}



export default AnchorLinks

```



This pattern also works for components. The benefit of doing this is that if you delete, or refactor the page, the anchor links associated with it will always stay with the page.



You could create a separate shared message component just for the anchor links but this would break the [proximity principle](https://kula.blog/posts/proximity_principle/).



A full example of anchor links can be found in the [example application](./example/pages/tests/anchor-links/).



### Search Engine Optimization



One feature that is missing from Next.js is managing important HTML tags used for SEO. We added the `` component to deal with two very important tags that live in the HTML ``:



- Canonical links (``): this tells search engines that the source of truth for the page being browsed is this URL. Very important to avoid being penalized for duplicate content, especially since URLs are case insensitive, but Google treats them as case-sensitive.

- Alternate links (``): this tells search engines that the page being browsed is also available in other languages and facilitates crawling of the site.



The API is available under `next-multilingual/head` and you can import it like this:



```js

import Head from 'next-multilingual/head'

```



Just like ``, `` is meant to be a drop-in replacement for Next.js' [`` component](https://nextjs.org/docs/api-reference/next/head). In our example, we are using it in the [Layout component](./example/src/components/layout/Layout.tsx), like this:



```tsx



  {title}

  



```



All this does is insert the canonical and alternate links so that search engines can better crawl your application. For example, if you are on the `/en-us/about-us` page, the following HTML will be added automatically under your HTML `` tag:



```html



```



To fully benefit from the SEO markup, `` must be included on all pages. There are multiple ways to achieve this, but in the example, we created a `` [component](./example/src/components/layout/Layout.tsx) that is used on all pages.



### Custom Error Pages



Like most sites, you will want to leverage Next.js' [custom error pages](https://nextjs.org/docs/advanced-features/custom-error-page) capability. With `useMessages()`, it's just as easy as creating any other pages. For example, for a `404` error, you can create your `404.tsx`:



```tsx

import { NextPage } from 'next'

import Link from 'next-multilingual/link'

import { getTitle, useMessages } from 'next-multilingual/messages'



import Layout from '@/layout'



const Error404: NextPage = () => {

  const messages = useMessages()

  const title = getTitle(messages)

  return (

    

      
{title}


      

        {messages.format('goBack')}

      

    

  )

}



export default Error404

```



And of course, your messages, for example `404.en-US.properties`:



```properties

# Page title

exampleApp.pageNotFoundError.title = 404 - Page Not Found

# Go back link text

exampleApp.pageNotFoundError.goBack = Go back home

```



### Using messages in APIs



APIs often need to be localized. Here is an "Hello API" example:



```ts

import type { NextApiRequest, NextApiResponse } from 'next'

import { getMessages } from 'next-multilingual/messages'



/**

 * Example API schema.

 */

type Schema = {

  message: string

}



/**

 * The "hello API" handler.

 */

const handler = (request: NextApiRequest, response: NextApiResponse): void => {

  const locale = request.headers['accept-language']



  if (locale === undefined || !isLocale(locale)) {

    response.status(400)

    return

  }



  const messages = getMessages(locale)

  response.status(200).json({ message: messages.format('message') })

}

```



This is very similar to the API implemented in the [example application](./example/pages/api). We are using the `Accept-Language` HTTP header to tell the API in which locale we want its response to be. Unlike the `useMessages` hook that has the context of the current locale, we need to tell `getMessages` in which locale to return messages.



Message files behave exactly the same as with `useMessages` You simply need to create one next to the API Route's file, in our case `hello.en-US.properties`:



```properties

# API message

exampleApp.helloApi.message = Hello, from API.

```



You can implement this in any pages, just like any other React-based API call, like this:



```tsx

const SomePage: NextPage = () => {

  const [apiError, setApiError] = useState(null)

  const [isApiLoaded, setApiIsLoaded] = useState(false)

  const [apiMessage, setApiMessage] = useState('')



  useEffect(() => {

    setApiIsLoaded(false)

    const requestHeaders: HeadersInit = new Headers()

    requestHeaders.set('Accept-Language', normalizeLocale(router.locale as string))

    fetch('/api/hello', { headers: requestHeaders })

      .then((result) => result.json())

      .then(

        (result) => {

          setApiIsLoaded(true)

          setApiMessage(result.message)

        },

        (apiError) => {

          setApiIsLoaded(true)

          setApiError(apiError)

        }

      )

  }, [router.locale])



  const showApiMessage: React.FC = () => {

    if (apiError) {

      return (

        <>

          {messages.format('apiError')}

          {(apiError as Error).message}

        >

      )

    } else if (!isApiLoaded) {

      return <>{messages.format('apiLoading')}>

    } else {

      return <>{apiMessage}>

    }

  }



  return (

    


      
{messages.format('apiHeader')}


      
{showApiMessage({})}


    


  )

}

```



The `normalizeLocale` is not mandatory but a recommended ISO 3166 convention. Since Next.js uses the locales as URL prefixes, they are lower-cased in the configuration and can be re-normalized as needed.



### Dynamic Routes



> ❗ Dynamic routes are complex and localizing them adds even more complexity. Make sure you are familiar with how this Next.js feature works before trying to add localization.



[Dynamic routes](https://nextjs.org/docs/routing/dynamic-routes) are very common and supported out of the box by Next.js. Since version 3.0, `next-multilingual` provides the same supports as Next.js in terms of dynamic routes. To make dynamic routes work with `next-multilingual` we have a few patterns to follow:



- The ``'s `href` attribute and the `useLocalizedUrl`/`useGetLocalizedUrl`/`getLocalizedUrl` `url` argument only accept string URLs.

  - Unlike Next.js' `` component which accepts a `UrlObject`, we preferred to streamline our types since `urlObject.href` can easily be used instead.

- Unlike static links where we expect the URL to be non-localized, the parameters always need to be localized inside the URL string by either using:

  - `userRouter().asPath` (most common scenario) by providing localized parameters directly in the URL. By using `asPath` you are using the localized URL which means that the URL you will use will be fully localized.

  - `userRouter().pathname` is conjunction with `hydrateRouteParameters` by providing localized parameters. By using `pathname` you are using the non-localized URL which means that the URL you will use might be a mix of non-localized segments plus the localized parameters. This can be useful in cases where you have nested dynamic routes.



We provided several examples of on on to use dynamic routes in our [dynamic route test pages](./example/pages/tests/dynamic-routes/).



#### Switching languages when using dynamic routes



The main challenge with dynamic routes, is that if the value of the parameter needs to be localized, we need to keep a relation between languages so that we can correctly switch languages. `next-multilingual` solves this problem with its `getLocalizedRouteParameters` API that creates a `LocalizedRouteParameters` object used as a page props. This can work both with `getStaticProps` and `getServerSideProps`.



#### Example using `getStaticProps`



First you need to tell Next.js which predefined paths will be valid by using `getStaticPaths` (all imports are added in the first example):



```tsx

import { getCitiesMessages } from '@/messages/cities/citiesMessages'

import { GetStaticPaths } from 'next'

import { slugify } from 'next-multilingual/messages'



export const getStaticPaths: GetStaticPaths = async (context) => {

  const paths: MultilingualStaticPath[] = []

  const { locales } = getStaticPathsLocales(context)

  locales.forEach((locale) => {

    const citiesMessages = getCitiesMessages(locale)

    citiesMessages.getAll().forEach((cityMessage) => {

      paths.push({

        params: {

          city: slugify(cityMessage.format(), locale),

        },

        locale,

      })

    })

  })

  return {

    paths,

    fallback: false,

  }

}

```



Then you have to pre-compute the localized route parameters and return them as props using `getStaticProps` and `getLocalizedRouteParameters`:



```ts

export type CityPageProps = { localizedRouteParameters: LocalizedRouteParameters }



export const getStaticProps: GetStaticProps = async (context) => {

  const localizedRouteParameters = getLocalizedRouteParameters(

    context,

    {

      city: getCitiesMessages,

    },

    import.meta.url

  )



  return { props: { localizedRouteParameters } }

}

```



If you are using a catch-all dynamic route, you will need to pass your parameters as an array, for each URL segment that you want to support. For example, if you want to support 2 levels:



```ts

const localizedRouteParameters = getLocalizedRouteParameters(context, {

  city: [getCitiesMessages, getCitiesMessages],

})

```



Note that since we need to use the `getMessages` API instead of the `useMessages` hook, you will also need to export it in the message file:



```ts

export {

  getMessages as getCitiesMessages,

  useMessages as useCitiesMessages,

} from 'next-multilingual/messages'

```



Finally you have to pass down your localized route parameters down to your language switcher component when you create your page:



```ts

const CityPage: NextPage = ({ localizedRouteParameters }) => {

  const messages = useMessages()

  const title = getTitle(messages)

  const { query } = useRouter()



  return (

    

      
{query['city']}


    

  )

}



export default CityPage

```



The only part missing now is the language switcher which needs to leverage the localized route parameters by using `getLanguageSwitcherUrl`:



```ts

import { normalizeLocale, setCookieLocale } from 'next-multilingual'

import Link from 'next-multilingual/link'

import { KeyValueObject } from 'next-multilingual/messages'

import { LocalizedRouteParameters, useRouter } from 'next-multilingual/router'

import { getLanguageSwitcherUrl } from 'next-multilingual/url'

import { ReactElement } from 'react'



// Locales don't need to be localized.

const localeStrings: KeyValueObject = {

  'en-US': 'English (United States)',

  'fr-CA': 'Français (Canada)',

}



type LanguageSwitcherProps = {

  /** Route parameters, if the page is using a dynamic route. */

  localizedRouteParameters?: LocalizedRouteParameters

}



export const LanguageSwitcher: React.FC = ({ localizedRouteParameters }) => {

  const router = useRouter()

  const { pathname, locale: currentLocale, locales, defaultLocale, query } = useRouter()

  const href = getLanguageSwitcherUrl(router, localizedRouteParameters)



  return (

    


      

    


  )

}

```



Check out our fully working examples:



- [Non-localizable dynamic route using a unique identifier](./example/pages/tests/dynamic-routes/identifier/[id].tsx)

- [Multi-level dynamic routes localized texts](./example/pages/tests/dynamic-routes/text/[city]/index.tsx[id].tsx)

- [Catch-all dynamic routes localized texts](./example/pages/tests/dynamic-routes/catch-all)



## Translation Process 🈺



Our ideal translation process is one where you send the modified files to your localization vendor (while working in a branch), and get back the translated files, with the correct locale in the filenames. Once you get the files back you basically submit them back in your branch which means localization becomes an integral part of the development process. Basically, the idea is:



- Don't modify the files, let the translation management system (TMS) do its job.

- Add a localization step in your development pipeline and wait for that step to be over before merging back to your main branch.



We don't have any "export/import" tool to help as at the time of writing this document.



## Why `next-multilingual`? πŸ—³οΈ



Why did we put so much effort into these details? Because our hypothesis is that it can have a major impact on:



- SEO

- Boosting customer trust with more locally relevant content.

- Making string management easier and more modular.



More details can be found on the implementation and design decision in the individual README files of each API and in the [documentation](./docs) directory.