Handle preferring default locale over accept-lang preferred locale (#17883)

This updates to set the `NEXT_LOCALE` cookie to the default locale when the user prefers a different locale from the default in their `accept-language` header but visits the default locale path e.g. `/en-US` with a `accept-language` preferred header of `nl` will set the `NEXT_LOCALE=en-US` header and then redirect to `/`

x-ref: https://github.com/vercel/next.js/pull/17370

packages/next/build/webpack/loaders/next-serverless-loader.ts

@@ -221,12 +221,17 @@ const nextServerlessLoader: loader.Loader = function () {
       // get pathname from URL with basePath stripped for locale detection
       const i18n = ${i18n}
       const accept = require('@hapi/accept')
+      const cookie = require('next/dist/compiled/cookie')
       const { detectLocaleCookie } = require('next/dist/next-server/lib/i18n/detect-locale-cookie')
       const { detectDomainLocale } = require('next/dist/next-server/lib/i18n/detect-domain-locale')
       const { normalizeLocalePath } = require('next/dist/next-server/lib/i18n/normalize-locale-path')
       let locales = i18n.locales
       let defaultLocale = i18n.defaultLocale
       let detectedLocale = detectLocaleCookie(req, i18n.locales)
+      let acceptPreferredLocale = accept.language(
+        req.headers['accept-language'],
+        i18n.locales
+      )
 
       const detectedDomain = detectDomainLocale(
         i18n.domains,
@@ -237,12 +242,8 @@ const nextServerlessLoader: loader.Loader = function () {
         detectedLocale = defaultLocale
       }
 
-      if (!detectedLocale) {
-        detectedLocale = accept.language(
-          req.headers['accept-language'],
-          i18n.locales
-        )
-      }
+      // if not domain specific locale use accept-language preferred
+      detectedLocale = detectedLocale || acceptPreferredLocale
 
       let localeDomainRedirect
       const localePathResult = normalizeLocalePath(parsedUrl.pathname, i18n.locales)
@@ -279,6 +280,7 @@ const nextServerlessLoader: loader.Loader = function () {
       const shouldStripDefaultLocale =
         detectedDefaultLocale &&
         denormalizedPagePath.toLowerCase() === \`/\${i18n.defaultLocale.toLowerCase()}\`
+
       const shouldAddLocalePrefix =
         !detectedDefaultLocale && denormalizedPagePath === '/'
 
@@ -294,6 +296,30 @@ const nextServerlessLoader: loader.Loader = function () {
           shouldStripDefaultLocale
         )
       ) {
+        // set the NEXT_LOCALE cookie when a user visits the default locale
+        // with the locale prefix so that they aren't redirected back to
+        // their accept-language preferred locale
+        if (
+          shouldStripDefaultLocale &&
+          acceptPreferredLocale !== defaultLocale
+        ) {
+          const previous = res.getHeader('set-cookie')
+
+          res.setHeader(
+            'set-cookie',
+            [
+            ...(typeof previous === 'string'
+              ? [previous]
+              : Array.isArray(previous)
+              ? previous
+              : []),
+            cookie.serialize('NEXT_LOCALE', defaultLocale, {
+              httpOnly: true,
+              path: '/',
+            })
+          ])
+        }
+
         res.setHeader(
           'Location',
           formatUrl({

packages/next/next-server/server/next-server.ts

@@ -80,6 +80,7 @@ import { normalizeLocalePath } from '../lib/i18n/normalize-locale-path'
 import { detectLocaleCookie } from '../lib/i18n/detect-locale-cookie'
 import * as Log from '../../build/output/log'
 import { detectDomainLocale } from '../lib/i18n/detect-domain-locale'
+import cookie from 'next/dist/compiled/cookie'
 
 const getCustomRouteMatcher = pathMatch(true)
 
@@ -310,6 +311,10 @@ export default class Server {
       const { pathname, ...parsed } = parseUrl(req.url || '/')
       let defaultLocale = i18n.defaultLocale
       let detectedLocale = detectLocaleCookie(req, i18n.locales)
+      let acceptPreferredLocale = accept.language(
+        req.headers['accept-language'],
+        i18n.locales
+      )
 
       const detectedDomain = detectDomainLocale(i18n.domains, req)
       if (detectedDomain) {
@@ -317,12 +322,8 @@ export default class Server {
         detectedLocale = defaultLocale
       }
 
-      if (!detectedLocale) {
-        detectedLocale = accept.language(
-          req.headers['accept-language'],
-          i18n.locales
-        )
-      }
+      // if not domain specific locale use accept-language preferred
+      detectedLocale = detectedLocale || acceptPreferredLocale
 
       let localeDomainRedirect: string | undefined
       const localePathResult = normalizeLocalePath(pathname!, i18n.locales)
@@ -360,6 +361,7 @@ export default class Server {
         detectedDefaultLocale &&
         denormalizedPagePath.toLowerCase() ===
           `/${i18n.defaultLocale.toLowerCase()}`
+
       const shouldAddLocalePrefix =
         !detectedDefaultLocale && denormalizedPagePath === '/'
 
@@ -371,6 +373,28 @@ export default class Server {
           shouldAddLocalePrefix ||
           shouldStripDefaultLocale)
       ) {
+        // set the NEXT_LOCALE cookie when a user visits the default locale
+        // with the locale prefix so that they aren't redirected back to
+        // their accept-language preferred locale
+        if (
+          shouldStripDefaultLocale &&
+          acceptPreferredLocale !== defaultLocale
+        ) {
+          const previous = res.getHeader('set-cookie')
+
+          res.setHeader('set-cookie', [
+            ...(typeof previous === 'string'
+              ? [previous]
+              : Array.isArray(previous)
+              ? previous
+              : []),
+            cookie.serialize('NEXT_LOCALE', defaultLocale, {
+              httpOnly: true,
+              path: '/',
+            }),
+          ])
+        }
+
         res.setHeader(
           'Location',
           formatUrl({

test/integration/i18n-support/test/index.test.js

@@ -132,6 +132,43 @@ function runTests(isDev) {
     expect(result2.query).toEqual({})
   })
 
+  it('should set locale cookie when removing default locale and accept-lang doesnt match', async () => {
+    const res = await fetchViaHTTP(appPort, '/en-US', undefined, {
+      headers: {
+        'accept-language': 'nl',
+      },
+      redirect: 'manual',
+    })
+
+    expect(res.status).toBe(307)
+
+    const parsedUrl = url.parse(res.headers.get('location'), true)
+    expect(parsedUrl.pathname).toBe('/')
+    expect(parsedUrl.query).toEqual({})
+    expect(res.headers.get('set-cookie')).toContain('NEXT_LOCALE=en-US')
+  })
+
+  it('should not redirect to accept-lang preferred locale with locale cookie', async () => {
+    const res = await fetchViaHTTP(appPort, '/', undefined, {
+      headers: {
+        'accept-language': 'nl',
+        cookie: 'NEXT_LOCALE=en-US',
+      },
+      redirect: 'manual',
+    })
+
+    expect(res.status).toBe(200)
+
+    const html = await res.text()
+    const $ = cheerio.load(html)
+
+    expect($('#router-locale').text()).toBe('en-US')
+    expect(JSON.parse($('#router-locales').text())).toEqual(locales)
+    expect($('html').attr('lang')).toBe('en-US')
+    expect($('#router-pathname').text()).toBe('/')
+    expect($('#router-as-path').text()).toBe('/')
+  })
+
   it('should redirect to correct locale domain', async () => {
     const checks = [
       // test domain, locale prefix, redirect result