refactor: make locale handling truly centralized and robust

- Added getPathWithoutLocale() and buildLocalePath() helpers to locales.ts
- Updated Header to use centralized helpers instead of hardcoded regex
- Updated middleware to use SUPPORTED_LOCALES in matcher config
- Updated LocaleProvider to use isValidLocale() instead of hardcoded array

To add a new language now, only update:
1. SUPPORTED_LOCALES in locales.ts
2. LOCALE_CONFIG entry with label, flag, saleorLocale
3. Add translation keys to all message files

All routing now uses centralized constants - no more hardcoded locale lists.

middleware.ts

@@ -1,25 +1,29 @@
 import { NextResponse } from "next/server";
 import type { NextRequest } from "next/server";
-import { SUPPORTED_LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE, getLocaleFromPath } from "@/lib/i18n/locales";
+import { SUPPORTED_LOCALES, DEFAULT_LOCALE, LOCALE_COOKIE, getPathWithoutLocale, buildLocalePath, isValidLocale } from "@/lib/i18n/locales";
+import type { Locale } from "@/lib/i18n/locales";
 
 const OLD_SERBIAN_PATHS = ["products", "about", "contact", "checkout"];
 
+function detectLocale(cookieLocale: string | undefined, acceptLanguage: string): Locale {
+  if (cookieLocale && isValidLocale(cookieLocale)) {
+    return cookieLocale;
+  }
+  if (acceptLanguage.includes("en")) {
+    return "en";
+  }
+  return DEFAULT_LOCALE;
+}
+
 export default function middleware(request: NextRequest) {
   const pathname = request.nextUrl.pathname;
   const cookieLocale = request.cookies.get(LOCALE_COOKIE)?.value;
   const acceptLanguage = request.headers.get("accept-language") || "";
 
   if (pathname === "/" || pathname === "") {
-    let locale = DEFAULT_LOCALE;
+    const locale = detectLocale(cookieLocale, acceptLanguage);
-
-    if (cookieLocale && SUPPORTED_LOCALES.includes(cookieLocale as typeof SUPPORTED_LOCALES[number])) {
-      locale = cookieLocale;
-    } else if (acceptLanguage.includes("en")) {
-      locale = "en";
-    }
-
     const url = request.nextUrl.clone();
-    url.pathname = `/${locale}`;
+    url.pathname = buildLocalePath(locale, "/");
     return NextResponse.redirect(url, 301);
   }
 
@@ -28,16 +32,10 @@ export default function middleware(request: NextRequest) {
   );
 
   if (isOldSerbianPath) {
-    let locale = DEFAULT_LOCALE;
+    const locale = detectLocale(cookieLocale, acceptLanguage);
-
+    const newPath = buildLocalePath(locale, pathname);
-    if (cookieLocale && SUPPORTED_LOCALES.includes(cookieLocale as typeof SUPPORTED_LOCALES[number])) {
-      locale = cookieLocale;
-    } else if (acceptLanguage.includes("en")) {
-      locale = "en";
-    }
-
     const url = request.nextUrl.clone();
-    url.pathname = `/${locale}${pathname}`;
+    url.pathname = newPath;
     return NextResponse.redirect(url, 301);
   }
 
@@ -47,7 +45,7 @@ export default function middleware(request: NextRequest) {
 export const config = {
   matcher: [
     "/",
-    "/(sr|en|de|fr)/:path*",
+    `/${SUPPORTED_LOCALES.join("|")}/:path*`,
     "/((?!api|_next|_vercel|.*\\..*).*)",
   ],
 };

src/components/layout/Header.tsx

@@ -9,7 +9,7 @@ import { useTranslations } from "next-intl";
 import { useSaleorCheckoutStore } from "@/stores/saleorCheckoutStore";
 import { User, ShoppingBag, Menu, X, Globe } from "lucide-react";
 import CartDrawer from "@/components/cart/CartDrawer";
-import { SUPPORTED_LOCALES, LOCALE_COOKIE, LOCALE_CONFIG, isValidLocale } from "@/lib/i18n/locales";
+import { SUPPORTED_LOCALES, LOCALE_COOKIE, LOCALE_CONFIG, isValidLocale, getPathWithoutLocale, buildLocalePath } from "@/lib/i18n/locales";
 import type { Locale } from "@/lib/i18n/locales";
 
 interface HeaderProps {
@@ -48,8 +48,8 @@ export default function Header({ locale = "sr" }: HeaderProps) {
       return;
     }
     document.cookie = `${LOCALE_COOKIE}=${newLocale}; path=/; max-age=31536000`;
-    const pathWithoutLocale = pathname.replace(/^\/(sr|en|de|fr)/, "") || "/";
+    const pathWithoutLocale = getPathWithoutLocale(pathname);
-    const newPath = `/${newLocale}${pathWithoutLocale}`;
+    const newPath = buildLocalePath(newLocale as Locale, pathWithoutLocale);
     window.location.replace(newPath);
     setLangDropdownOpen(false);
   };

src/components/providers/LocaleProvider.tsx

@@ -3,6 +3,7 @@
 import { NextIntlClientProvider } from "next-intl";
 import { getMessages } from "next-intl/server";
 import { notFound } from "next/navigation";
+import { SUPPORTED_LOCALES, isValidLocale } from "@/lib/i18n/locales";
 
 export default async function LocaleProvider({
   children,
@@ -11,8 +12,7 @@ export default async function LocaleProvider({
   children: React.ReactNode;
   locale: string;
 }) {
-  const locales = ["en", "sr"];
+  if (!isValidLocale(locale)) notFound();
-  if (!locales.includes(locale)) notFound();
 
   const messages = await getMessages();
 

src/lib/i18n/locales.ts

@@ -21,6 +21,19 @@ export function getSaleorLocale(locale: Locale): string {
 }
 
 export function getLocaleFromPath(pathname: string): string {
-  const match = pathname.match(/^\/(sr|en|de|fr)/);
+  const pattern = SUPPORTED_LOCALES.join("|");
+  const match = pathname.match(new RegExp(`^\\/(${pattern})`));
   return match ? match[1] : DEFAULT_LOCALE;
 }
+
+export function getPathWithoutLocale(pathname: string): string {
+  const pattern = SUPPORTED_LOCALES.join("|");
+  return pathname.replace(new RegExp(`^\\/(${pattern})`), "") || "/";
+}
+
+export function buildLocalePath(locale: Locale, path: string): string {
+  if (locale === DEFAULT_LOCALE) {
+    return path === "/" ? "/" : path;
+  }
+  return `/${locale}${path === "/" ? "" : path}`;
+}