Next.js 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'],
 
  // Used when no locale matches
  defaultLocale: 'en'
});
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

In addition to handling i18n routing, the middleware sets the link header (opens in a new tab) to inform search engines that your content is available in different languages.

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.

Strategy 1: Prefix-based routing (default)

Since your pages are nested within a [locale] folder, all routes are by default prefixed with one of your supported locales (e.g. /en/about).

Locale detection

The locale is detected based on these priorities:

1. A locale prefix is present in the pathname (e.g. /en/about)
2. A cookie is present that contains a previously detected locale
3. A locale can be matched based on the accept-language header (opens in a new tab)
4. As a last resort, 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 and will update the previous cookie value.

Example workflow:

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

Strategy 2: 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.

Examples:

• us.example.com/en
• ca.example.com/en
• ca.example.com/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.
    }
  ]
});

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

Locale prefix

Always use a locale prefix

By default, pathnames always start with the locale (e.g. /en/about).

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'always' // This is the default
});

Don't use a locale prefix for the default locale

If you don't want to include a locale prefix for the default locale, but only for non-default locales, you can configure the middleware accordingly.

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

In this case, requests where the locale prefix matches the default locale will be redirected (e.g. /en/about to /about). This will affect both prefix-based as well as domain-based routing.

Important: If you use this strategy, you should make sure that your matcher detects unprefixed pathnames.

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'
});

In this case, requests for all locales will be rewritten to have the locale only prefixed internally. You still need to place all your pages inside a [locale] folder for the routes to be able to receive the locale param.

Important: If you use this strategy, you should make sure that your matcher detects unprefixed pathnames.

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
});

In this case, only the locale prefix and a potentially matching domain are used to determine the locale.

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, 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

Many apps choose to localize pathnames, especially when search engine optimization is relevant, e.g.:

• /en/about
• /de/ueber-uns

Since you want to define these routes only once internally, you can use the next-intl middleware to rewrite (opens in a new tab) such incoming requests to shared pathnames.

import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  defaultLocale: 'en',
  locales: ['en', 'de'],
 
  // The `pathnames` object holds pairs of internal and
  // external paths. Based on the locale, the external
  // paths are rewritten to the shared, internal ones.
  pathnames: {
    // If all locales use the same pathname, a single
    // external path can be used for all locales.
    '/': '/',
    '/blog': '/blog',
 
    // If locales use different paths, you can
    // specify each external path per locale.
    '/about': {
      en: '/about',
      de: '/ueber-uns'
    },
 
    // Dynamic params are supported via square brackets
    '/news/[articleSlug]-[articleId]': {
      en: '/news/[articleSlug]-[articleId]',
      de: '/neuigkeiten/[articleSlug]-[articleId]'
    },
 
    // Also (optional) catch-all segments are supported
    '/categories/[...slug]': {
      en: '/categories/[...slug]',
      de: '/kategorien/[...slug]'
    }
  }
});

Matcher config

The middleware is intended to only run on pages, not on arbitrary files that you serve independently of the user locale (e.g. /favicon.ico).

Because of this, the following config is generally recommended:

export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

This enables:

1. A redirect at / to a suitable locale
2. Internationalization of all pathnames starting with a locale (e.g. /en/about)

Pathnames without a locale prefix

If you pick a config for localePrefix other than always, you need a more flexible matcher since you have to match pathnames without a locale prefix as well (e.g. /about).

A popular strategy is to match all routes that don't start with certain segments (e.g. /_next) and also none that include a dot (.) since these typically are static files. However, if you have some routes where a dot is expected (e.g. /users/jane.doe), you should explicitly provide a matcher for these.

export const config = {
  // Matcher entries are linked with a logical "or", therefore
  // if one of them matches, the middleware will be invoked.
  matcher: [
    // Match all pathnames except for
    // - … if they start with `/api`, `/_next` or `/_vercel`
    // - … the ones containing a dot (e.g. `favicon.ico`)
    '/((?!api|_next|_vercel|.*\\..*).*)',
    // Match all pathnames within `/users`, optionally with a locale prefix
    '/(.+)?/users/(.+)'
  ]
};

Note that some third-party providers like Vercel Analytics (opens in a new tab) and umami (opens in a new tab) typically use internal endpoints that are then rewritten to an external URL (e.g. /_vercel/insights/view). Make sure to exclude such requests from your middleware matcher so they aren't accidentally rewritten.

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 (example)
  const defaultLocale = request.headers.get('x-default-locale') || 'en';
 
  // Step 2: Create and call the next-intl middleware (example)
  const handleI18nRouting = createIntlMiddleware({
    locales: ['en', 'de'],
    defaultLocale
  });
  const response = handleI18nRouting(request);
 
  // Step 3: Alter the response (example)
  response.headers.set('x-default-locale', defaultLocale);
 
  return response;
}
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

Example: Additional rewrites

If you need to handle rewrites apart from the ones provided by next-intl, you can adjust the pathname of the request before invoking the next-intl middleware (based on "A/B Testing with Cookies" by Vercel (opens in a new tab)).

This example rewrites requests for /[locale]/profile to /[locale]/profile/new if a special cookie is set.

import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
export default async function middleware(request: NextRequest) {
  const [, locale, pathname] = request.nextUrl.pathname.split('/');
 
  if (pathname === 'profile') {
    const usesNewProfile =
      (request.cookies.get('NEW_PROFILE')?.value || 'false') === 'true';
 
    if (usesNewProfile) {
      request.nextUrl.pathname = `/${locale}/profile/new`;
    }
  }
 
  const handleI18nRouting = createIntlMiddleware({
    locales: ['en', 'de'],
    defaultLocale: 'en'
  });
  const response = handleI18nRouting(request);
  return response;
}
 
export const config = {
  matcher: ['/', '/(de|en)/:path*']
};

Example: Integrating with Clerk

@clerk/nextjs (opens in a new tab) provides a middleware that can be integrated with next-intl by using the beforeAuth hook (opens in a new tab). By doing this, the middleware from next-intl will run first, potentially redirect or rewrite incoming requests, followed by the middleware from @clerk/next acting on the response.

import {authMiddleware} from '@clerk/nextjs';
import createMiddleware from 'next-intl/middleware';
 
const intlMiddleware = createMiddleware({
  locales: ['en', 'de'],
  defaultLocale: 'en'
});
 
export default authMiddleware({
  beforeAuth(request) {
    return intlMiddleware(request);
  },
 
  // Ensure that locale-specific sign in pages are public
  publicRoutes: ['/:locale', '/:locale/sign-in']
});
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

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,
  localePrefix: 'as-necssary',
  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|.*\\..*).*)']
};

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 can't use pathname localization
4. This requires static rendering
5. 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 `/` is requested
export default function RootPage() {
  redirect('/en');
}

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

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

Troubleshooting

"Unable to find next-intl locale because the middleware didn't run on this request."

This can happen either because:

1. The middleware is not set up.
2. The middleware is set up in the wrong file (e.g. you're using the src folder, but middleware.ts was added in the root folder).
3. The middleware matcher didn't match a request, but you're using APIs from next-intl in a component.
4. You're attempting to implement static rendering via force-static (opens in a new tab).

To recover from this error, please make sure that:

1. You've set up the middleware.
2. You're using APIs from next-intl (including the navigation APIs) exclusively within the locale segment.
3. Your middleware matcher matches all routes of your application, including dynamic segments with potentially unexpected characters like dots (e.g. /users/jane.doe).
4. If you're using localePrefix: 'as-needed', the locale segment effectively acts like a catch-all for all unknown routes. You should make sure that params.locale is validated before it's used by any APIs from next-intl.
5. To implement static rendering properly, make sure to provide a static locale to next-intl.