Ensure correct default locale is detected for domain locale

packages/next/build/webpack-config.ts

@@ -1012,9 +1012,12 @@ export default async function getBaseWebpackConfig(
         'process.env.__NEXT_IMAGE_OPTS': JSON.stringify(config.images),
         'process.env.__NEXT_ROUTER_BASEPATH': JSON.stringify(config.basePath),
         'process.env.__NEXT_HAS_REWRITES': JSON.stringify(hasRewrites),
-        'process.env.__NEXT_i18n_SUPPORT': JSON.stringify(
+        'process.env.__NEXT_I18N_SUPPORT': JSON.stringify(
           !!config.experimental.i18n
         ),
+        'process.env.__NEXT_I18N_DOMAINS': JSON.stringify(
+          config.experimental.i18n.domains
+        ),
         'process.env.__NEXT_ANALYTICS_ID': JSON.stringify(
           config.experimental.analyticsId
         ),

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

@@ -233,9 +233,13 @@ const nextServerlessLoader: loader.Loader = function () {
         i18n.locales
       )
 
+      const { host } = req.headers || {}
+      // remove port from host and remove port if present
+      const hostname = host && host.split(':')[0].toLowerCase()
+
       const detectedDomain = detectDomainLocale(
         i18n.domains,
-        req,
+        hostname,
       )
       if (detectedDomain) {
         defaultLocale = detectedDomain.defaultLocale
@@ -291,6 +295,7 @@ const nextServerlessLoader: loader.Loader = function () {
       if (
         !fromExport &&
         !nextStartMode &&
+        !req.headers["${vercelHeader}"] &&
         i18n.localeDetection !== false &&
         (
           localeDomainRedirect ||

packages/next/client/index.tsx

@@ -83,11 +83,15 @@ if (hasBasePath(asPath)) {
   asPath = delBasePath(asPath)
 }
 
-if (process.env.__NEXT_i18n_SUPPORT) {
+if (process.env.__NEXT_I18N_SUPPORT) {
   const {
     normalizeLocalePath,
   } = require('../next-server/lib/i18n/normalize-locale-path')
 
+  const {
+    detectDomainLocale,
+  } = require('../next-server/lib/i18n/detect-domain-locale')
+
   if (locales) {
     const localePathResult = normalizeLocalePath(asPath, locales)
 
@@ -99,6 +103,18 @@ if (process.env.__NEXT_i18n_SUPPORT) {
       // locales
       defaultLocale = locale
     }
+
+    // attempt detecting default locale based on hostname
+    const detectedDomain = detectDomainLocale(
+      process.env.__NEXT_I18N_DOMAINS,
+      window.location.hostname
+    )
+
+    // TODO: investigate if defaultLocale needs to be populated after
+    // hydration to prevent mismatched renders
+    if (detectedDomain) {
+      defaultLocale = detectedDomain.defaultLocale
+    }
   }
 }
 

packages/next/next-server/lib/i18n/detect-domain-locale.ts

@@ -1,5 +1,3 @@
-import { IncomingMessage } from 'http'
-
 export function detectDomainLocale(
   domainItems:
     | Array<{
@@ -8,7 +6,7 @@ export function detectDomainLocale(
         defaultLocale: string
       }>
     | undefined,
-  req?: IncomingMessage,
+  hostname?: string,
   detectedLocale?: string
 ) {
   let domainItem:
@@ -20,10 +18,6 @@ export function detectDomainLocale(
     | undefined
 
   if (domainItems) {
-    const { host } = req?.headers || {}
-    // remove port from host and remove port if present
-    const hostname = host?.split(':')[0].toLowerCase()
-
     for (const item of domainItems) {
       // remove port if present
       const domainHostname = item.domain?.split(':')[0].toLowerCase()

packages/next/next-server/lib/router/router.ts

@@ -61,7 +61,7 @@ export function addLocale(
   locale?: string,
   defaultLocale?: string
 ) {
-  if (process.env.__NEXT_i18n_SUPPORT) {
+  if (process.env.__NEXT_I18N_SUPPORT) {
     return locale && locale !== defaultLocale && !path.startsWith('/' + locale)
       ? addPathPrefix(path, '/' + locale)
       : path
@@ -70,7 +70,7 @@ export function addLocale(
 }
 
 export function delLocale(path: string, locale?: string) {
-  if (process.env.__NEXT_i18n_SUPPORT) {
+  if (process.env.__NEXT_I18N_SUPPORT) {
     return locale && path.startsWith('/' + locale)
       ? path.substr(locale.length + 1) || '/'
       : path
@@ -453,7 +453,7 @@ export default class Router implements BaseRouter {
 
     this.isFallback = isFallback
 
-    if (process.env.__NEXT_i18n_SUPPORT) {
+    if (process.env.__NEXT_I18N_SUPPORT) {
       this.locale = locale
       this.locales = locales
       this.defaultLocale = defaultLocale

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

@@ -318,7 +318,11 @@ export default class Server {
         i18n.locales
       )
 
-      const detectedDomain = detectDomainLocale(i18n.domains, req)
+      const { host } = req?.headers || {}
+      // remove port from host and remove port if present
+      const hostname = host?.split(':')[0].toLowerCase()
+
+      const detectedDomain = detectDomainLocale(i18n.domains, hostname)
       if (detectedDomain) {
         defaultLocale = detectedDomain.defaultLocale
         detectedLocale = defaultLocale
@@ -564,9 +568,12 @@ export default class Server {
           const { i18n } = this.nextConfig.experimental
 
           if (i18n) {
+            const { host } = req?.headers || {}
+            // remove port from host and remove port if present
+            const hostname = host?.split(':')[0].toLowerCase()
             const localePathResult = normalizeLocalePath(pathname, i18n.locales)
             const { defaultLocale } =
-              detectDomainLocale(i18n.domains, req) || {}
+              detectDomainLocale(i18n.domains, hostname) || {}
             let detectedLocale = defaultLocale
 
             if (localePathResult.detectedLocale) {