feat: implement language-based homepage redirection

- Add custom middleware for automatic language detection
- Map Chinese language codes (zh, zh-CN, zh-TW, zh-HK) to 'cn'
- Redirect non-default language users to localized paths (/cn/)
- Keep default language (en) users on clean URLs (/)
- Store language preference in cookies for consistency
- Add negotiator and type definitions as dependencies

Co-authored-by: huangyiirene <7665279+huangyiirene@users.noreply.github.com>

Author: Copilot

apps/docs/middleware.ts

@@ -1,15 +1,121 @@
-import { createI18nMiddleware } from 'fumadocs-core/i18n/middleware';
+import { NextRequest, NextResponse } from 'next/server';
+import Negotiator from 'negotiator';
 import { i18n } from '@/lib/i18n';
 
+const LOCALE_COOKIE = 'FD_LOCALE';
+
+/**
+ * Language code mapping
+ * Maps browser language codes to our supported language codes
+ */
+const LANGUAGE_MAPPING: Record<string, string> = {
+  'zh': 'cn',      // Chinese -> cn
+  'zh-CN': 'cn',   // Chinese (China) -> cn
+  'zh-TW': 'cn',   // Chinese (Taiwan) -> cn
+  'zh-HK': 'cn',   // Chinese (Hong Kong) -> cn
+};
+
+/**
+ * Normalize language code to match our supported languages
+ */
+function normalizeLanguage(lang: string): string {
+  // Check direct mapping first
+  if (LANGUAGE_MAPPING[lang]) {
+    return LANGUAGE_MAPPING[lang];
+  }
+  
+  // Check if the base language (without region) is mapped
+  const baseLang = lang.split('-')[0];
+  if (LANGUAGE_MAPPING[baseLang]) {
+    return LANGUAGE_MAPPING[baseLang];
+  }
+  
+  return lang;
+}
+
+/**
+ * Get the preferred language from the request
+ */
+function getPreferredLanguage(request: NextRequest): string {
+  // Check cookie first
+  const cookieLocale = request.cookies.get(LOCALE_COOKIE)?.value;
+  if (cookieLocale && (i18n.languages as readonly string[]).includes(cookieLocale)) {
+    return cookieLocale;
+  }
+
+  // Then check Accept-Language header
+  const negotiatorHeaders: Record<string, string> = {};
+  request.headers.forEach((value, key) => {
+    negotiatorHeaders[key] = value;
+  });
+
+  const negotiator = new Negotiator({ headers: negotiatorHeaders });
+  const browserLanguages = negotiator.languages();
+  
+  // Normalize browser languages to match our supported languages
+  const normalizedLanguages = browserLanguages.map(normalizeLanguage);
+  
+  // Find the first match
+  for (const lang of normalizedLanguages) {
+    if ((i18n.languages as readonly string[]).includes(lang)) {
+      return lang;
+    }
+  }
+  
+  return i18n.defaultLanguage;
+}
+
 /**
  * Middleware for automatic language detection and redirection
  * 
  * This middleware:
  * - Detects the user's preferred language from browser settings or cookies
  * - Redirects users to the appropriate localized version
+ * - For default language (en): keeps URL as "/" (with internal rewrite)
+ * - For other languages (cn): redirects to "/cn/"
  * - Stores language preference as a cookie
  */
-export default createI18nMiddleware(i18n);
+export default function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  // Check if the pathname already has a locale
+  const pathnameHasLocale = i18n.languages.some(
+    (locale) => pathname.startsWith(`/${locale}/`) || pathname === `/${locale}`
+  );
+
+  if (pathnameHasLocale) {
+    // Extract the locale from the pathname
+    const locale = pathname.split('/')[1];
+    
+    // If it's the default locale and hideLocale is 'default-locale', redirect to remove locale prefix
+    if (locale === i18n.defaultLanguage && i18n.hideLocale === 'default-locale') {
+      const url = new URL(request.url);
+      url.pathname = pathname.replace(`/${i18n.defaultLanguage}`, '') || '/';
+      const response = NextResponse.redirect(url);
+      response.cookies.set(LOCALE_COOKIE, locale);
+      return response;
+    }
+    
+    return NextResponse.next();
+  }
+
+  // Pathname doesn't have a locale, determine preferred language
+  const preferredLanguage = getPreferredLanguage(request);
+
+  // If preferred language is the default, rewrite internally (keep URL clean)
+  if (preferredLanguage === i18n.defaultLanguage && i18n.hideLocale === 'default-locale') {
+    const url = new URL(request.url);
+    url.pathname = `/${i18n.defaultLanguage}${pathname}`;
+    return NextResponse.rewrite(url);
+  }
+
+  // For non-default languages, redirect to the localized path
+  const url = new URL(request.url);
+  url.pathname = `/${preferredLanguage}${pathname}`;
+  const response = NextResponse.redirect(url);
+  response.cookies.set(LOCALE_COOKIE, preferredLanguage);
+  return response;
+}
 
 export const config = {
   // Match all routes except:

apps/docs/package.json

@@ -21,12 +21,15 @@
     "server-only": "^0.0.1"
   },
   "devDependencies": {
+    "@formatjs/intl-localematcher": "^0.8.0",
     "@tailwindcss/postcss": "^4.1.18",
     "@tailwindcss/typography": "^0.5.19",
+    "@types/negotiator": "^0.6.4",
     "@types/node": "^20.10.0",
     "@types/react": "^19.2.8",
     "@types/react-dom": "^19.2.3",
     "autoprefixer": "^10.4.23",
+    "negotiator": "^1.0.0",
     "postcss": "^8.5.6",
     "tailwindcss": "^4.0.0",
     "typescript": "^5.3.0",