Next.js 13 middleware for i18n routing

The middleware handles redirects and rewrites based on the detected user locale.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // A list of all locales that are supported
  locales: ['en', 'de'],
 
  // If this locale is matched, pathnames work without a prefix (e.g. `/about`)
  defaultLocale: 'en'
});
 
export const config = {
  // Skip all paths that should not be internationalized. This example skips the
  // folders "api", "_next" and all files with an extension (e.g. favicon.ico)
  matcher: ['/((?!api|_next|.*\\..*).*)']
};

Strategies

There are two strategies for detecting the locale:

1. Prefix-based routing (default)
2. Domain-based routing

Once a locale is detected, it will be saved in a cookie.

Prefix-based routing (default)

Since your pages are nested within a [locale] folder, all routes are prefixed with one of your supported locales (e.g. /de/about). To keep the URL short, requests for the default locale are rewritten internally to work without a locale prefix.

Request examples:

• / → /en
• /about → /en/about
• /de/about → /de/about

Locale detection

The locale is detected based on these priorities:

1. A locale prefix is present in the pathname (e.g. /de/about)
2. A cookie is present that contains a previously detected locale
3. The accept-language header (opens in a new tab) is matched against the available locales
4. The defaultLocale is used

To change the locale, users can visit a prefixed route. This will take precedence over a previously matched locale that is saved in a cookie or the accept-language header.

Example workflow:

1. A user requests / and based on the accept-language header, the de locale is matched.
2. The de locale is saved in a cookie and the user is redirected to /de.
3. The app renders <Link locale="en" href="/">Switch to English</Link> to allow the user to change the locale to en.
4. When the user clicks on the link, a request to /en is initiated.
5. The middleware will update the cookie value to en and subsequently redirects the user to /.

Domain-based routing

If you want to serve your localized content based on different domains, you can provide a list of mappings between domains and locales to the middleware.

Example:

• us.example.com (default: en)
• ca.example.com (default: en)
• ca.example.com/fr (fr)

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // All locales across all domains
  locales: ['en', 'fr'],
 
  // Used when no domain matches (e.g. on localhost)
  defaultLocale: 'en',
 
  domains: [
    {
      domain: 'us.example.com',
      defaultLocale: 'en',
      // Optionally restrict the locales managed by this domain. If this
      // domain receives requests for another locale (e.g. us.example.com/fr),
      // then the middleware will redirect to a domain that supports it.
      locales: ['en']
    },
    {
      domain: 'ca.example.com',
      defaultLocale: 'en'
      // If there are no `locales` specified on a domain,
      // all global locales will be supported here.
    }
  ]
});

The middleware rewrites the requests internally, so that requests for the defaultLocale on a given domain work without a locale prefix (e.g. us.example.com/about → /en/about). If you want to include a prefix for the default locale as well, you can add localePrefix: 'always'.

Locale detection

To match the request against the available domains, the host is read from the x-forwarded-host header, with a fallback to host.

The locale is detected based on these priorities:

1. A locale prefix is present in the pathname and the domain supports it (e.g. ca.example.com/fr)
2. If the host of the request is configured in domains, the defaultLocale of the domain is used
3. As a fallback, the locale detection of prefix-based routing applies

Since the middleware is aware of all your domains, the domain will automatically be switched when the user requests to change the locale.

Example workflow:

1. The user requests us.example.com and based on the defaultLocale of this domain, the en locale is matched.
2. The app renders <Link locale="fr" href="/">Switch to French</Link> to allow the user to change the locale to fr.
3. When the link is clicked, a request to us.example.com/fr is initiated.
4. The middleware recognizes that the user wants to switch to another domain and responds with a redirect to ca.example.com/fr.

Further configuration

Always use a locale prefix

If you want to include a prefix for the default locale as well, you can configure the middleware accordingly.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'always' // Defaults to 'as-needed'
});

In this case, requests without a prefix will be redirected accordingly (e.g. /about to /en/about).

Note that this will affect both prefix-based as well as domain-based routing.

Never use a locale prefix

For applications behind an authentication layer, where there is no need for SEO, it is possible to have the locale never show up in the URL.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'never' // Defaults to 'as-needed'
});

In this case all requests for all locales will be rewritten internally.

Disable automatic locale detection

If you want to rely entirely on the URL to resolve the locale, you can disable locale detection based on the accept-language header and a potentially existing cookie value from a previous visit.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localeDetection: false // Defaults to `true`
});

Note that in this case other detection mechanisms will remain in place regardless (e.g. based on a locale prefix in the pathname or a matched domain).

Disable alternate links

The middleware automatically sets the link header (opens in a new tab) to inform search engines that your content is available in different languages. Note that this automatically integrates with your routing strategy and will generate the correct links based on your configuration.

If you prefer to include these links yourself, e.g. because you're using locale-specific rewrites, you can opt-out of this behavior.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  alternateLinks: false // Defaults to `true`
});

Localizing pathnames

If you want to localize the pathnames of your app, you can accomplish this by using appropriate rewrites (opens in a new tab).

const withNextIntl = require('next-intl/plugin')();
 
module.exports = withNextIntl({
  rewrites() {
    return [
      {
        source: '/de/über',
        destination: '/de/about'
      }
    ];
  }
});

Since next-intl isn't aware of the rewrites you've configured, you likely want to make some adjustments:

1. Translate the pathnames you're passing to navigation APIs like Link based on the locale. See the named routes example (opens in a new tab) that uses the proposed APIs from the Server Components beta (opens in a new tab).
2. Turn off the alternateLinks option and provide search engine hints about localized versions of your content (opens in a new tab) by yourself.

Composing other middlewares

By calling createMiddleware, you'll receive a function of the following type:

middleware(request: NextRequest): NextResponse

If you need to incorporate additional behavior, you can either modify the request before the next-intl middleware receives it, or modify the response that is returned.

import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
export default async function middleware(request: NextRequest) {
  // Step 1: Use the incoming request
  const defaultLocale = request.headers.get('x-default-locale') || 'en';
 
  // Step 2: Create and call the next-intl middleware
  const handleI18nRouting = createIntlMiddleware({
    locales: ['en', 'de'],
    defaultLocale
  });
  const response = handleI18nRouting(request);
 
  // Step 3: Alter the response
  response.headers.set('x-default-locale', defaultLocale);
 
  return response;
}
 
export const config = {
  matcher: ['/((?!_next|.*\\..*).*)']
};

Example: Integrating with Auth.js (aka NextAuth.js)

The Next.js middleware of Auth.js (opens in a new tab) requires an integration with their control flow to be compatible with other middlewares. The success callback (opens in a new tab) can be used to run the next-intl middleware on authorized pages. However, public pages need to be treated separately.

For pathnames specified in the pages object (opens in a new tab) (e.g. signIn), Auth.js will skip the entire middleware and not run the success callback. Therefore, we have to detect these pages before running the Auth.js middleware and only run the next-intl middleware in this case.

import {withAuth} from 'next-auth/middleware';
import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
const locales = ['en', 'de'];
const publicPages = ['/', '/login'];
 
const intlMiddleware = createIntlMiddleware({
  locales,
  defaultLocale: 'en'
});
 
const authMiddleware = withAuth(
  // Note that this callback is only invoked if
  // the `authorized` callback has returned `true`
  // and not for pages listed in `pages`.
  function onSuccess(req) {
    return intlMiddleware(req);
  },
  {
    callbacks: {
      authorized: ({token}) => token != null
    },
    pages: {
      signIn: '/login'
    }
  }
);
 
export default function middleware(req: NextRequest) {
  const publicPathnameRegex = RegExp(
    `^(/(${locales.join('|')}))?(${publicPages.join('|')})?/?$`,
    'i'
  );
  const isPublicPage = publicPathnameRegex.test(req.nextUrl.pathname);
 
  if (isPublicPage) {
    return intlMiddleware(req);
  } else {
    return (authMiddleware as any)(req);
  }
}
 
export const config = {
  matcher: ['/((?!api|_next|.*\\..*).*)']
};

Many thanks to narakhan (opens in a new tab) for sharing his middleware implementation (opens in a new tab)!

Usage without middleware (static export)

If you're using the static export feature from Next.js (opens in a new tab) (output: 'export'), the middleware will not run. You can use prefix-based routing nontheless to internationalize your app, but a few tradeoffs apply.

Static export limitations:

1. There's no default locale that can be used without a prefix (same as localePrefix: 'always')
2. The locale can't be negotiated at runtime (same as localeDetection: false)
3. You need to add a redirect for the root of the app

import {redirect} from 'next/navigation';
 
// Redirect the user to the default locale when the app root is requested
export default function RootPage() {
  redirect('/en');
}

You can explore a working demo by building the Next.js 13 example after enabling the static export:

module.exports = {
  output: 'export'
};