next-multilingual

next-multilingual is an opinionated end-to-end solution for Next.js for 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.
• All URLs will use a locale prefix - this is currently a limitation of Next.js where the default locale does not use a prefix.
• Smart language detection that dynamically renders the homepage, without using redirections.
• The ability to use localized URLs (e.g. /en-us/contact-us for U.S. English and /fr-ca/nous-joindre for Canadian French).
• Automatically generate canonical and alternate links optimized for SEO.
• Modular localized string configuration support that works just like CSS (no more files containing shared strings).

Before we start 💎

next-multilingual has put a lot of effort to add JSDoc 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 tto ensure that your messages (localized strings) have unique identifiers.
• Your locales: we only support BCP47 language tags that contains both a country and language code.

We offer two APIs to simplify this step:

〰️ getMulConfig (simple config)

Short for "get multilingual configuration", this function will generate a Next.js config that will meet most use cases. getMulConfig takes the following arguments:

• applicationIdentifier — 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.
• Also a few other arguments you probably will never need to use - check in your IDE (JSDoc) for more details.

getMulConfig will return a Next.js configuration object.

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

const { getMulConfig } = require('next-multilingual/config');
module.exports = getMulConfig('exampleApp', ['en-US', 'fr-CA'], { poweredByHeader: false });

Some options are not supported by getMulConfig. If you try to use one, the error message should point you directly to the next section: advanced config.

〰️ MulConfig (advanced config)

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

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

const mulConfig = new MulConfig('exampleApp', ['en-US', 'fr-CA']);

module.exports = {
    i18n: {
        locales: mulConfig.getUrlLocalePrefixes(),
        defaultLocale: mulConfig.getDefaultUrlLocalePrefix(),
        localeDetection: false
    },
    poweredByHeader: false,
    webpack(config, { isServer }) {
        if (isServer) {
            config.resolve.alias['next-multilingual/link$'] = require.resolve('next-multilingual/link/ssr');
        }
        return config;
    },
    async rewrites() {
        return mulConfig.getRewrites();
    },
    async redirects() {
        return mulConfig.getRedirects();
    }
};

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 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 { useRouter } from 'next/router';
import { getActualDefaultLocale, setCookieLocale } from 'next-multilingual';

export default function MyApp({ Component, pageProps }: AppProps): JSX.Element {
  const router = useRouter();
  const { locales, defaultLocale, locale } = router;
  /**
   * Next.js always expose the default locale with URLs without prefixes. If anyone use these URLs, we want to overwrite them
   * with the actual (default) locale.
   */
  if (locale === defaultLocale) {
    router.locale = getActualDefaultLocale(locales, defaultLocale);
  }
  setCookieLocale(router.locale); // Persist locale on page load (will be re-used when hitting `/`).

  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. Persist the actual locale in the cookie so we can reuse it when hitting the homepage without a locale (/).

You might have noticed the getActualDefaultLocale API. This API is part of a set of "utility" APIs that helps abstract some of the complexity that we configured in Next.js. These APIs are very important, since we can no longer rely on the locales provided by Next.js. The main reason for this is that we set the default Next.js locale to mul (for multilingual) to allow us to do the dynamic detection on the homepage. These APIs are simple and more details are available in your IDE (JSDoc).

Create a custom Document (_document.tsx)

We also need to create a custom Document by adding _document.tsx in the pages directory:

import Document, { Html, Head, Main, NextScript } from 'next/document';
import { ReactElement } from 'react';
import { getActualLocale, normalizeLocale } from 'next-multilingual';

class MyDocument extends Document {
  render(): ReactElement {
    const { locale, locales, defaultLocale, props } = this.props.__NEXT_DATA__;
    const pagePropsActualLocale = props?.pageProps?.resolvedLocale;
    const actualLocale = pagePropsActualLocale
      ? pagePropsActualLocale
      : getActualLocale(locale, defaultLocale, locales);

    return (
      <Html lang={normalizeLocale(actualLocale)}>
        <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 /. 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.

Configure all your pages to use SEO friendly markup

next-multilingual/head provides a <MulHead> 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!

Creating the homepage

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

• Redirecting on / can have negative SEO impact 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 {
  getActualLocales,
  getActualDefaultLocale,
  getActualLocale,
  getPreferredLocale,
  getCookieLocale
} from 'next-multilingual';
import type { NextPageContext } from 'next';
import { useRouter } from 'next/router';
import { ReactElement } from 'react';
import Layout from '@/layout';
import { useMessages } from 'next-multilingual/messages';
import {
  ResolvedLocaleServerSideProps,
  setCookieLocale
} from 'next-multilingual';

export default function IndexPage({
  resolvedLocale
}: ResolvedLocaleServerSideProps): ReactElement {
  const router = useRouter();

  // Overwrite the locale with the resolved locale.
  router.locale = resolvedLocale;
  setCookieLocale(router.locale);

  // Load the messages in the correct locale.
  const messages = useMessages();

  return (
    <Layout title={messages.format('pageTitle')}>
      <h1>{messages.format('headline')}</h1>
    </Layout>
  );
}

export async function getServerSideProps(
  nextPageContext: NextPageContext
): Promise<{ props: ResolvedLocaleServerSideProps }> {
  const { req, locale, locales, defaultLocale } = nextPageContext;

  const actualLocales = getActualLocales(locales, defaultLocale);
  const actualDefaultLocale = getActualDefaultLocale(locales, defaultLocale);
  const cookieLocale = getCookieLocale(nextPageContext, actualLocales);
  let resolvedLocale = getActualLocale(locale, defaultLocale, locales);

  // When Next.js tries to use the default locale, try to find a better one.
  if (locale === defaultLocale) {
    resolvedLocale = cookieLocale
      ? cookieLocale
      : getPreferredLocale(
          req.headers['accept-language'],
          actualLocales,
          actualDefaultLocale
        ).toLowerCase();
  }

  return {
    props: {
      resolvedLocale
    }
  };
}

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 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 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 to another file's local scope. This has the benefit to make messages more modular and also not share messages across different context (more details in the design decisions document on why this is bad).

There are two uses cases for message files:

• For the message's files in your pages directory, they will determine what the URL segment (part of a URL in between /) of this page is using the pageTitle key identifier. On top of that, they will be used as local scope messages if there are messages required specific to that page.
• They will store all the localizable strings (messages) used by your application. Note that you should only put the message used in the page directly since components also have their own message files. Those messages will be used by the useMessages hook and will only be available in local scopes. Imagine CSS but for localizable stings.

To summarize:

• Messages are associated to a 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.
• To leverage some of the built-in IDE support for .properties files, we follow a strict naming convention: <Page-Name>.<locale>.properties
• Each message must have unique keys that follow a strict naming convention: <application identifier>.<context>.<id> where:
  • application identifier 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 3 to 50 alphanumerical 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 pageTitle identifier.
• 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 component" (more details on 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

Also, as mentioned previously, there is one special key for pages, where the id is pageTitle. This message will be used both as a page title, but also as the localized URL segment of that page. Basically the "page title" is the human readable "short description" of your page, and represents a segment (contained between slashes) of a URL. When used as a URL segment, following changes are 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 pageTitle will not be used to create URL segments.

⚠️ Note that if you change pageTitle, this means that the 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). We don't really recommend this as this will make URL paths longer which goes against SEO best practice. But the option remains in case it is necessary.

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 (will not be used as a URL segment)
exampleApp.homepage.pageTitle = 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 { useMessages } from 'next-multilingual/messages';
import type { ReactElement } from 'react';
import Layout from '@/layout';

export default function AboutUs(): ReactElement {
  const messages = useMessages();
  return (
    <Layout title={messages.format('pageTitle')}>
      <h1>{messages.format('pageTitle')}</h1>
      <p>{messages.format('details')}</p>
    </Layout>
  );
}

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

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

Adding links

next-multilingual comes with its own <MulLink> 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 <MulLink> 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 { MulLink } from 'next-multilingual/link';

export default function ContactUs() {
  return (
    <>
      <MulLink href="/contact-us">
        <a>Contact us</a>
      </MulLink>
    </>
  );
}

In English the URL path will be /en-us/contact-us. But when another locale is selected, you will get the localized URLs path. See the example below for when fr-ca is selected:

<a href="/fr-ca/nous-joindre"></a>

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 getMulConfig, the Webpack configuration will be added automatically. If you are using the advanced MulConfig method, this explains on why the special Webpack configuration is required in the example provided prior.

Creating components

Creating components is the same as pages but they live outside the pages folder. Also as mentioned previously you do not need to add the pageTitle key. We have a few example components that should be self explanatory but here is an example of a Footer.tsx component:

import type { ReactElement } from 'react';
import { useMessages } from 'next-multilingual/messages';

export default function Footer(): ReactElement {
  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 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 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 list of items.

For example, to keep your localization process simple, you want to avoid as much as possible 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 component that calls useMessages like this:

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

export const useFruitsMessages = useMessages;

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 component from anywhere you might need these values:

import type { ReactElement } from 'react';
import { useFruitsMessages } from '../messages/Fruits';

export default function FruitList(): ReactElement {
  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 Variables

Using variables 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, {firstName} {lastName}!

And inject back the values using:

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

If you do not provide the values of your variables when formatting the message, it will simply output the message as static text.

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 an 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 corret 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.

Search Engine Optimization

One feature that is missing from Next.js is manage important HTML tags used for SEO. We added the <MulHead> 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 { MulHead } from 'next-multilingual/head';

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

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

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, <MulHead> 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 site, 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 { useMessages } from 'next-multilingual/messages';
import { MulLink } from 'next-multilingual/link';
import type { ReactElement } from 'react';
import Layout from '@/layout';

export default function Custom400(): ReactElement {
  const messages = useMessages();
  return (
    <Layout title={messages.format('pageTitle')}>
      <h1>{messages.format('pageTitle')}</h1>
      <MulLink href="/">
        <a>{messages.format('goBack')}</a>
      </MulLink>
    </Layout>
  );
}

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

exampleApp.pageNotFoundError.pageTitle = 404 - Page Not Found
exampleApp.pageNotFoundError.goBack = Go back home

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 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 with 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 an be found on the implementation and design decision in the individual README files of each API and in the documentation directory.