feat(i18n): make language a user profile attribute (register/profile/login)

src/middleware.ts

@@ -6,6 +6,20 @@ import { routing } from "@/i18n/routing";
 
 const intlMiddleware = createIntlMiddleware(routing);
 
+// One-time marker: set after we've applied the user's profile language
+// once following sign-in, cleared whenever the login page is shown (so
+// the next sign-in re-applies it). Keeps the header switcher a
+// per-session override rather than forcing the profile locale on every
+// navigation.
+const LOCALE_INIT_COOKIE = "pieced_locale_init";
+
+const LOCALE_INIT_OPTS = {
+  path: "/",
+  httpOnly: true,
+  sameSite: "lax" as const,
+  maxAge: 8 * 60 * 60,
+};
+
 const publicPaths = ["/login", "/register", "/api/auth", "/api/register"];
 
 function isPublicPath(pathname: string): boolean {
@@ -26,6 +40,17 @@ export default async function middleware(request: NextRequest) {
     return NextResponse.next();
   }
 
+  const stripped = pathname.replace(/^\/(de|fr|it|en)(?=\/|$)/, "") || "/";
+
+  // Showing the login page resets the one-time locale marker so the
+  // next sign-in re-applies the user's profile language. Logout
+  // redirects here, which makes this the natural reset point.
+  if (stripped === "/login") {
+    const res = intlMiddleware(request);
+    res.cookies.delete(LOCALE_INIT_COOKIE);
+    return res;
+  }
+
   // Auth guard for protected paths
   if (!isPublicPath(pathname)) {
     const session = await auth();
@@ -34,6 +59,32 @@ export default async function middleware(request: NextRequest) {
       loginUrl.searchParams.set("callbackUrl", pathname);
       return NextResponse.redirect(loginUrl);
     }
+
+    // One-time apply of the user's preferred language after sign-in.
+    // Gated by LOCALE_INIT_COOKIE (cleared on the /login view), so it
+    // fires at most once per login; afterwards the URL and the header
+    // switcher control the locale freely.
+    const applied = request.cookies.get(LOCALE_INIT_COOKIE)?.value === "1";
+    const pref = (session as { platformUser?: { locale?: string } })
+      .platformUser?.locale;
+    const base = pref?.split("-")[0];
+    if (!applied && base && routing.locales.includes(base as never)) {
+      const target =
+        base === routing.defaultLocale
+          ? stripped
+          : `/${base}${stripped === "/" ? "" : stripped}`;
+      if (target !== pathname) {
+        const url = new URL(target, request.url);
+        url.search = request.nextUrl.search;
+        const res = NextResponse.redirect(url);
+        res.cookies.set(LOCALE_INIT_COOKIE, "1", LOCALE_INIT_OPTS);
+        return res;
+      }
+      // Already on the right locale — mark applied and continue.
+      const res = intlMiddleware(request);
+      res.cookies.set(LOCALE_INIT_COOKIE, "1", LOCALE_INIT_OPTS);
+      return res;
+    }
   }
 
   return intlMiddleware(request);