next-multilingual

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

Check out our demo app!

Installation 💻

npm install next-multilingual

What's in it for me? 🤔

• 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 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).
• 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.

Before we start 💎

next-multilingual has put a lot of effort into adding TSDoc 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. 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 💨

For those who prefer to jump right into the action, look in the 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 ⚙️

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 — The unique application identifier that will be used as a messages key prefix.
• locales — The actual desired locales of the multilingual application. The first locale will be the default locale. Only BCP 47 language tags following the language-country format are accepted. For more details on why, refer to the design decisions document.
• options (optional) — Options part of a Next.js configuration object.

getConfig will return a Next.js configuration object.

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

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

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

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

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

module.exports = {
  reactStrictMode: true,
  i18n: {
    locales: config.getUrlLocalePrefixes(),
    defaultLocale: config.getDefaultUrlLocalePrefix(),
    localeDetection: false,
  },
  poweredByHeader: false,
  /* This is required since Next.js 11.1.3-canary.69 until we support ESM. */
  experimental: {
    esmExternals: false,
  },
  webpack: webpackConfigurationHandler,
}

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

import Webpack from 'webpack'

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

export function myWebpackConfigurationHandler(
  config: Webpack.Configuration,
  context: WebpackContext
): Webpack.Configuration {
  const myConfig = webpackConfigurationHandler(config, context)
  // Do stuff here.
  return myConfig
}

Or directly in next.config.js:

// Webpack handler wrapping next-multilingual's handler.
function 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 to link localized URLs to the default language URLs.
2. Add Redirects to redirect all possible encoded URL forms to the normalized NFC 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 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 plugin that will automatically inject strings into pages and components. The recommended way to do this is to include a .babelrc at the base of your application:

{
  "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 by adding _app.tsx in the pages directory:

import type { AppProps } from 'next/app'
import { useActualLocale } from 'next-multilingual'

export default function MyApp({ Component, pageProps }: AppProps): JSX.Element {
  useActualLocale() // Forces Next.js to use the actual (proper) locale.
  return <Component {...pageProps} />
}

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 by adding _document.tsx in the pages directory:

import { getHtmlLang } from 'next-multilingual'
import Document, { Head, Html, Main, NextScript } from 'next/document'

class MyDocument extends Document {
  render(): JSX.Element {
    return (
      <Html lang={getHtmlLang(this)} translate="no" className="notranslate">
        <Head>
          <meta name="google" content="notranslate" />
        </Head>
        <body>
          <Main />
          <NextScript />
        </body>
      </Html>
    )
  }
}

export default MyDocument

This serves only 1 purpose: display the correct server side locale in the <html> 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 <Head> 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, 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. 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):

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, 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, 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.

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

const Page: NextPage = () => {
  const router = useRouter()
  return <>{router.locale}</>
}

export default Page

getStaticPropsLocales

import { getStaticPropsLocales } from 'next-multilingual'

export const getStaticProps: GetStaticProps = async (context) => {
  const { locale, locales, defaultLocale } = getStaticPropsLocales(context)
  // do stuff
  return { props: {} }
}

getStaticPathsLocales

import { getStaticPathsLocales } from 'next-multilingual'

export const getStaticPaths: GetStaticProps = async (context) => {
  const { locales, defaultLocale } = getStaticPathsLocales(context)
  // do stuff
  return { props: {} }
}

getServerSidePropsLocales

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, but here is a stripped down version:

import type { GetServerSideProps, NextPage } from 'next'
import { ResolvedLocaleServerSideProps, resolveLocale, useResolvedLocale } from 'next-multilingual'
import { useRouter } from 'next-multilingual/router'

const Homepage: NextPage<ResolvedLocaleServerSideProps> = ({ 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 <h1>{locale}</h1>
}

export default Homepage

export const getServerSideProps: GetServerSideProps<ResolvedLocaleServerSideProps> = 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 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.
• Make sure your file encoding is set to UTF-8. Not doing so will replace non-Latin characters with �.
• To leverage some of the built-in IDE support for .properties files, we follow a strict naming convention: <PageFilename>.<locale>.properties
• Each message must have unique keys that follow a strict naming convention: <applicationId>.<context>.<id> 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 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
• spaces 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.<locale>.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 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 to see message files in action, but here is a sample that could be used on the homepage:

# 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, but here is a sample of what about-us.jsx could look like:

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 (
    <Layout title={title}>
      <h1 className={styles.headline}>{title}</h1>
      <p>{messages.format('details')}</p>
    </Layout>
  )
}

export default AboutUs

And of course you would have this message file about-us.en-US.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 <Link> component that allows for client side and server side rendering of localized URL. It's usage is simple, it works exactly like Next.js' <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 <Link> 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:

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

export default function Menu(): JSX.Element {
  const messages = useMessages()

  return (
    <nav>
      <Link href="/">
        <a>{messages.format('home')}</a>
      </Link>
      <Link href="/about-us">
        <a>{messages.format('aboutUs')}</a>
      </Link>
      <Link href="/contact-us">
        <a>{messages.format('contactUs')}</a>
      </Link>
    </nav>
  )
}

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 <Link> 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:

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 <button onClick={() => router.push(localizedUrl)}>{messages.format('clickMe')}</button>
}

export default Tests

Server side localized URLs

There could be cases where you need to use localized URLs on the server side and hooks (useLocalizedUrl) cannot be used. 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. This is where getLocalizedUrl comes in. getLocalizedUrl is only usable on the server side which is why it is imported directly from next-multilingual/url/ssr. Here is an example of how it can be used:

import type { NextApiRequest, NextApiResponse } from 'next'
import { isLocale } from 'next-multilingual'
import { getLocalizedUrl } from 'next-multilingual/url/ssr'
import { getMessages } from 'next-multilingual/messages'

import { sendEmail } from '../send-email/'

/**
 * The "/api/send-email" handler.
 */
export default async function handler(
  request: NextApiRequest,
  response: NextApiResponse
): Promise<void> {
  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: getLocalizedUrl('/login', locale, true) })
  )
  response.status(200)
}

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 that should be self-explanatory but here is an example of a Footer.tsx component:

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

export default function Footer(): JSX.Element {
  const messages = useMessages()
  return <footer>{messages.format('footerMessage')}</footer>
}

And its messages file:

# This is the message in the footer at the bottom of pages
exampleApp.footerComponent.footerMessage = © Footer

Also make sure to look at the language picker component example 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). 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. All you need to do, is create a hook that calls useMessages like this:

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

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

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:

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

export default function FruitList(): JSX.Element {
  const fruitsMessages = useFruitsMessages()
  return (
    <>
      {fruitsMessages
        .getAll()
        .map((message) => message.format())
        .join(', ')}
    </>
  )
}

You can also call individual messages like this:

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 syntax out of the box which means that you can use the following message:

exampleApp.homepage.welcome = Hello {name}!

And inject back the values using:

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 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:

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:

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 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:

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:

# 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:

<div>
  {messages.format('createAccount1')}
  <Link href="/sign-up">{messages.format('createAccount2')}</Link>
  {messages.format('createAccount3')}
</div>

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:

exampleApp.homepage.createAccount = Please <link>create your account</link> today for free.

And then:

<div>{messages.formatJsx('createAccount', { link: <Link href="/sign-up"></Link> })}</div>

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 <link> XML tag, the JSX element needs to be provided using link: <Link href="/"></Link>.
5. XML tag names must be unique in a message and cannot be repeated. This means that even if you use <i> many times in a sentence, you will need to create unique tags like <i1>, <i2>, 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>{name}</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 <Link href="/contact-us><a id="test"></a></Link> is valid but <div><span1></span1><span2></span2></div> 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 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 <element></element> (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:

exampleApp.statsPage.targetAchieved = You achieved your weekly target (&#x3c;5) and are eligible for a <link>reward</link>.

Anchor Links

Anchor links 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-équipe.

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:

# 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:

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 (
    <div>
      <div>
        <h2>{messages.format('tableOfContent')}</h2>
        <ul>
          <li>
            <Link href={`#${slugify(messages.format('p1Header'), locale)}`}>
              {messages.format('p1Header')}
            </Link>
          </li>
        </ul>
      </div>
      <div>
        <h2 id={slugify(messages.format('p1Header'), locale)}>{messages.format('p1Header')}</h2>
        <p>{messages.format('p1')}</p>
      </div>
    </div>
  )
}

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":

export const useLongPageMessages = useMessages

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

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 (
    <div>
      <div>
        <Link
          href={`${pathname}/long-page#${slugify(longPageMessages.format('p3Header'), locale)}`}
        >
          {messages.format('linkAction')}
        </Link>
      </div>
    </div>
  )
}

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.

A full example of anchor links can be found in the example application.

Search Engine Optimization

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

• Canonical links (<link rel=canonical>): 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 (<link rel=alternate>): 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:

import Head from 'next-multilingual/head'

Just like <Link>, <Head> is meant to be a drop-in replacement for Next.js' <Head> component. In our example, we are using it in the Layout component, like this:

<Head>
  <title>{title}</title>
  <meta name="viewport" content="width=device-width, initial-scale=1.0"></meta>
</Head>

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 <head> tag:

<link rel="canonical" href="http://localhost:3000/en-us/about-us" />
<link rel="alternate" href="http://localhost:3000/en-us/about-us" hreflang="en-US" />
<link rel="alternate" href="http://localhost:3000/fr-ca/%C3%A0-propos-de-nous" hreflang="fr-CA" />

To fully benefit from the SEO markup, <Head> must be included on all pages. There are multiple ways to achieve this, but in the example, we created a <Layout> component that is used on all pages.

Custom Error Pages

Like most sites, you will want to leverage Next.js' custom error pages 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:

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

import Layout from '@/layout'

const Custom400: NextPage = () => {
  const messages = useMessages()
  const title = getTitle(messages)
  return (
    <Layout title={title}>
      <h1>{title}</h1>
      <Link href="/">
        <a>{messages.format('goBack')}</a>
      </Link>
    </Layout>
  )
}

export default Custom400

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

# Page title
exampleApp.pageNotFoundError.title = 404 - Page Not Found
# Go back link text
exampleApp.pageNotFoundError.goBack = Go back home

Localized API Routes

One of Next.js' core features is its builtin API support. It's not uncommon for APIs to return content in different languages. next-multilingual has an equivalent API just for this use case: getMessages. Unlike the useMessages hook, getMessages can be used in API Routes. Here is an "Hello API" example on how to use it:

import type { NextApiRequest, NextApiResponse } from 'next'
import { getMessages } from 'next-multilingual/messages'

/**
 * Example API schema.
 */
type Schema = {
  message: string
}

/**
 * The "hello API" handler.
 */
export default function handler(request: NextApiRequest, response: NextApiResponse<Schema>): 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. 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:

# 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:

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])

  function showApiMessage(): JSX.Element {
    if (apiError) {
      return (
        <>
          {messages.format('apiError')}
          {(apiError as Error).message}
        </>
      )
    } else if (!isApiLoaded) {
      return <>{messages.format('apiLoading')}</>
    } else {
      return <>{apiMessage}</>
    }
  }

  return (
    <div>
      <h2>{messages.format('apiHeader')}</h2>
      <div>{showApiMessage()}</div>
    </div>
  )
}

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 very common and supported out of the box by Next.js. For simplicity, next-multilingual currently only supports path matching which is also the most common dynamic route use case. To make dynamic routes work with next-multilingual we use UrlObject instead of string when passing URLs. Just like any other links, we want to pass the non-localized path used by the Next.js' router (pathname) but we also need to include the route parameters by using the query properties like this:

<Link href={{ pathname: '/user/[id]', query: { id: '123' } }} />

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):

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:

export type CityPageProps = { localizedRouteParameters: LocalizedRouteParameters }

export const getStaticProps: GetStaticProps<CityPageProps> = async (context) => {
  const localizedRouteParameters = getLocalizedRouteParameters(context, {
    city: getCitiesMessages,
  })

  return { props: { localizedRouteParameters } }
}

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

const CityPage: NextPage<CityPageProps> = ({ localizedRouteParameters }) => {
  const messages = useMessages()
  const title = getTitle(messages)
  const { query } = useRouter()

  return (
    <Layout title={title} localizedRouteParameters={localizedRouteParameters}>
      <h1>{query['city']}</h1>
    </Layout>
  )
}

export default CityPage

The only part missing now is the language picker which needs to leverage the localized route parameters:

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 { ReactElement } from 'react'

// Locales don't need to be localized.
const localeStrings: KeyValueObject = {
  'en-US': 'English (United States)',
  'fr-CA': 'Français (Canada)',
}

type LanguagePickerProps = {
  /** Route parameters, if the page is using a dynamic route. */
  localizedRouteParameters?: LocalizedRouteParameters
}

export const LanguagePicker: React.FC<LanguagePickerProps> = ({
  localizedRouteParameters,
}): ReactElement => {
  const { pathname, locale: currentLocale, locales, defaultLocale, query } = useRouter()

  return (
    <div id="language-picker">
      <ul>
        {locales
          .filter((locale) => locale !== currentLocale)
          .map((locale) => {
            const parameters =
              (localizedRouteParameters && localizedRouteParameters[locale]) ?? query
            return (
              <li key={locale}>
                <Link href={{ pathname, query: parameters }} locale={locale}>
                  <a
                    onClick={() => {
                      setCookieLocale(locale)
                    }}
                    lang={normalizeLocale(locale)}
                  >
                    {localeStrings[normalizeLocale(locale)]}
                  </a>
                </Link>
              </li>
            )
          })}
      </ul>
    </div>
  )
}

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:

export {
  getMessages as getCitiesMessages,
  useMessages as useCitiesMessages,
} from 'next-multilingual/messages'

Check out our fully working examples:

• Non-localizable dynamic route using a unique identifier
• Multi-level dynamic routes localized texts

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? 🗳️

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 directory.

Changelog

2.0.0 (2022-09-18)

Features

• simplified APIs to access locale values (ac2dcb3)

BREAKING CHANGES

• replaced the first few getLocalizedRouteParameters arguments to take directly the getStaticProps or getServerSideProps contexts.
• useRouter will now return the locales used by next-multilingual instead of those used by Next.js:
  • getActualLocale, getActualLocales and getActualDefaultLocale should not longer be required
• replacing MultilingualStaticProps by getStaticPropsLocales
• replacing MultilingualServerSideProps by getServerSidePropsLocales