Make sure locale detecting is case-insensitive

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

@@ -241,10 +241,10 @@ const nextServerlessLoader: loader.Loader = function () {
       }
 
       const denormalizedPagePath = denormalizePagePath(parsedUrl.pathname || '/')
-      const detectedDefaultLocale = !detectedLocale || detectedLocale === defaultLocale
+      const detectedDefaultLocale = !detectedLocale || detectedLocale.toLowerCase() === defaultLocale.toLowerCase()
       const shouldStripDefaultLocale =
         detectedDefaultLocale &&
-        denormalizedPagePath === \`/\${defaultLocale}\`
+        denormalizedPagePath.toLowerCase() === \`/\${defaultLocale.toLowerCase()}\`
       const shouldAddLocalePrefix =
         !detectedDefaultLocale && denormalizedPagePath === '/'
 

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

@@ -1,15 +1,10 @@
 import { IncomingMessage } from 'http'
 
 export function detectLocaleCookie(req: IncomingMessage, locales: string[]) {
-  let detectedLocale: string | undefined
-
   if (req.headers.cookie && req.headers.cookie.includes('NEXT_LOCALE')) {
     const { NEXT_LOCALE } = (req as any).cookies
-
-    if (locales.some((locale: string) => NEXT_LOCALE === locale)) {
-      detectedLocale = NEXT_LOCALE
-    }
+    return locales.find(
+      (locale: string) => NEXT_LOCALE.toLowerCase() === locale.toLowerCase()
+    )
   }
-
-  return detectedLocale
 }

packages/next/next-server/lib/i18n/normalize-locale-path.ts

@@ -10,7 +10,7 @@ export function normalizeLocalePath(
   const pathnameParts = pathname.split('/')
 
   ;(locales || []).some((locale) => {
-    if (pathnameParts[1] === locale) {
+    if (pathnameParts[1].toLowerCase() === locale.toLowerCase()) {
       detectedLocale = locale
       pathnameParts.splice(1, 1)
       pathname = pathnameParts.join('/') || '/'

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

@@ -324,9 +324,11 @@ export default class Server {
 
       const denormalizedPagePath = denormalizePagePath(pathname || '/')
       const detectedDefaultLocale =
-        !detectedLocale || detectedLocale === defaultLocale
+        !detectedLocale ||
+        detectedLocale.toLowerCase() === defaultLocale.toLowerCase()
       const shouldStripDefaultLocale =
-        detectedDefaultLocale && denormalizedPagePath === `/${defaultLocale}`
+        detectedDefaultLocale &&
+        denormalizedPagePath.toLowerCase() === `/${defaultLocale.toLowerCase()}`
       const shouldAddLocalePrefix =
         !detectedDefaultLocale && denormalizedPagePath === '/'
 

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

@@ -173,6 +173,15 @@ function runTests() {
       expect($('#router-as-path').text()).toBe('/gsp')
       expect($('#router-pathname').text()).toBe('/gsp')
       expect(JSON.parse($('#router-locales').text())).toEqual(locales)
+
+      // make sure locale is case-insensitive
+      const html2 = await renderViaHTTP(appPort, `/${locale.toUpperCase()}/gsp`)
+      const $2 = cheerio.load(html2)
+      expect($2('html').attr('lang')).toBe(locale)
+      expect($2('#router-locale').text()).toBe(locale)
+      expect($2('#router-as-path').text()).toBe('/gsp')
+      expect($2('#router-pathname').text()).toBe('/gsp')
+      expect(JSON.parse($2('#router-locales').text())).toEqual(locales)
     }
   })
 
@@ -193,6 +202,21 @@ function runTests() {
 
     expect(parsedUrl.pathname).toBe('/')
     expect(parsedUrl.query).toEqual({})
+
+    // make sure locale is case-insensitive
+    const res2 = await fetchViaHTTP(appPort, '/eN-Us', undefined, {
+      redirect: 'manual',
+      headers: {
+        'Accept-Language': 'en-US;q=0.9',
+      },
+    })
+
+    expect(res2.status).toBe(307)
+
+    const parsedUrl2 = url.parse(res.headers.get('location'), true)
+
+    expect(parsedUrl2.pathname).toBe('/')
+    expect(parsedUrl2.query).toEqual({})
   })
 
   it('should load getStaticProps page correctly SSR (default locale no prefix)', async () => {
@@ -391,8 +415,8 @@ function runTests() {
   it('should navigate client side for default locale with no prefix', async () => {
     const browser = await webdriver(appPort, '/')
     // make sure default locale is used in case browser isn't set to
-    // favor en-US by default
-    await browser.manage().addCookie({ name: 'NEXT_LOCALE', value: 'en-US' })
+    // favor en-US by default, (we use all caps to ensure it's case-insensitive)
+    await browser.manage().addCookie({ name: 'NEXT_LOCALE', value: 'EN-US' })
     await browser.get(browser.initUrl)
 
     const checkIndexValues = async () => {