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

src/app/[locale]/register/page.tsx

@@ -1,7 +1,7 @@
 "use client";
 
 import { useState, useRef, forwardRef } from "react";
-import { useTranslations } from "next-intl";
+import { useTranslations, useLocale } from "next-intl";
 import { useRouter, Link } from "@/i18n/navigation";
 import { Card } from "@/components/ui/card";
 
@@ -41,11 +41,17 @@ export default function RegisterPage() {
 
   const [accountType, setAccountType] = useState<AccountType | null>(null);
 
+  const locale = useLocale();
+
   const [form, setForm] = useState({
     companyName: "",
     givenName: "",
     familyName: "",
     email: "",
+    // Default to the language the register page is being viewed in;
+    // the user can change it below. This becomes their ZITADEL
+    // preferredLanguage and the UI language they land on after login.
+    preferredLanguage: locale,
   });
   const [state, setState] = useState<FormState>("idle");
   const [error, setError] = useState("");
@@ -94,6 +100,7 @@ export default function RegisterPage() {
         givenName: form.givenName,
         familyName: form.familyName,
         email: form.email,
+        preferredLanguage: form.preferredLanguage,
         isPersonal,
       };
       if (!isPersonal) {
@@ -295,6 +302,29 @@ export default function RegisterPage() {
                 />
               </div>
 
+              {/* Preferred language */}
+              <div>
+                <label className="block text-xs font-semibold uppercase tracking-wider text-text-muted mb-1.5">
+                  {t("languageLabel")}
+                </label>
+                <select
+                  name="preferredLanguage"
+                  value={form.preferredLanguage}
+                  onChange={(e) =>
+                    setForm((prev) => ({
+                      ...prev,
+                      preferredLanguage: e.target.value,
+                    }))
+                  }
+                  className="w-full px-3 py-2 bg-surface-2 border border-border rounded-lg text-sm text-text-primary focus:outline-none focus:ring-1 focus:ring-accent focus:border-accent transition-colors"
+                >
+                  <option value="de">Deutsch</option>
+                  <option value="en">English</option>
+                  <option value="fr">Français</option>
+                  <option value="it">Italiano</option>
+                </select>
+              </div>
+
               {error && (
                 <div className="text-xs text-red-400 bg-red-400/10 border border-red-400/20 rounded-lg px-3 py-2">
                   {error}

src/app/[locale]/settings/profile/page.tsx

@@ -30,13 +30,14 @@ export default async function ProfileSettingsPage() {
 
   const t = await getTranslations("settingsProfile");
 
-  let initial = { firstName: "", lastName: "", email: user.email };
+  let initial = { firstName: "", lastName: "", email: user.email, language: "" };
   try {
     const profile = await getHumanUserDetail(user.id);
     initial = {
       firstName: profile.givenName,
       lastName: profile.familyName,
       email: profile.email || user.email,
+      language: profile.preferredLanguage,
     };
   } catch (e) {
     // Identity provider unreachable: render the form with whatever

src/app/api/settings/profile/route.ts

@@ -26,6 +26,7 @@ import {
 const updateSchema = z.object({
   firstName: z.string().trim().min(1).max(100),
   lastName: z.string().trim().min(1).max(100),
+  language: z.enum(["de", "en", "fr", "it"]).optional(),
 });
 
 export async function GET() {
@@ -66,6 +67,7 @@ export async function PUT(request: Request) {
       userId: user.id,
       givenName: parsed.data.firstName,
       familyName: parsed.data.lastName,
+      preferredLanguage: parsed.data.language,
     });
     return NextResponse.json({
       displayName: result.displayName,

src/components/settings/profile-form.tsx

@@ -2,7 +2,7 @@
 
 import { useState } from "react";
 import { useSession } from "next-auth/react";
-import { useTranslations } from "next-intl";
+import { useTranslations, useLocale } from "next-intl";
 import { Card } from "@/components/ui/card";
 
 interface Props {
@@ -10,6 +10,8 @@ interface Props {
     firstName: string;
     lastName: string;
     email: string;
+    /** Current ZITADEL preferredLanguage; "" if never set. */
+    language: string;
   };
   /**
    * Personal-account flag. Drives a small hint about how the ZITADEL
@@ -43,10 +45,15 @@ interface Props {
  */
 export function ProfileSettingsForm({ initial, isPersonal, orgName }: Props) {
   const t = useTranslations("settingsProfile");
+  const locale = useLocale();
   const { update } = useSession();
   const [form, setForm] = useState({
     firstName: initial.firstName,
     lastName: initial.lastName,
+    // Fall back to the current UI locale when the profile has no stored
+    // preference yet (older accounts), so the selector shows something
+    // sensible rather than blank.
+    language: initial.language || locale,
   });
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
@@ -67,6 +74,7 @@ export function ProfileSettingsForm({ initial, isPersonal, orgName }: Props) {
         body: JSON.stringify({
           firstName: form.firstName.trim(),
           lastName: form.lastName.trim(),
+          language: form.language,
         }),
       });
       const data = await res.json().catch(() => ({}));
@@ -79,15 +87,15 @@ export function ProfileSettingsForm({ initial, isPersonal, orgName }: Props) {
       // to session.user.name. No re-login needed.
       await update({ name: data.displayName });
       setSavedFlash(true);
-      // Force a full reload so EVERY server-rendered component picks
+      // If the language changed, land the user on the new locale (a
-      // up the new session cookie immediately — router.refresh() only
+      // full navigation so every server-rendered surface re-renders in
-      // re-runs the current route's server components, leaving the
+      // the new language). Otherwise just reload so the new name
-      // nav-shell (rendered higher in the tree) and other cached
+      // propagates. The 800ms delay lets the "Saved" flash show first.
-      // segments showing the old name until the user navigates.
+      const localeChanged = form.language && form.language !== locale;
-      // The 800ms delay lets the "Saved" flash render briefly before
+      const target = localeChanged ? localePath(form.language) : null;
-      // the page reloads, so the user gets visible feedback.
       setTimeout(() => {
-        window.location.reload();
+        if (target) window.location.assign(target);
+        else window.location.reload();
       }, 800);
     } catch (e: any) {
       setError(e?.message ?? String(e));
@@ -132,6 +140,20 @@ export function ProfileSettingsForm({ initial, isPersonal, orgName }: Props) {
             className="w-full px-3 py-2 rounded-md bg-surface-2 border border-border text-sm text-text-muted cursor-not-allowed"
           />
         </Field>
+        <Field label={t("languageLabel")} hint={t("languageHint")}>
+          <select
+            value={form.language}
+            onChange={(e) =>
+              setForm((f) => ({ ...f, language: e.target.value }))
+            }
+            className="w-full px-3 py-2 rounded-md bg-surface-2 border border-border focus:border-accent focus:outline-none text-sm"
+          >
+            <option value="de">Deutsch</option>
+            <option value="en">English</option>
+            <option value="fr">Français</option>
+            <option value="it">Italiano</option>
+          </select>
+        </Field>
         {/* Personal vs company hint. Personals get the
             "this won't change your invoice name" warning since their
             ZITADEL name and their invoice identity are intentionally
@@ -163,6 +185,15 @@ export function ProfileSettingsForm({ initial, isPersonal, orgName }: Props) {
   );
 }
 
+// Build the as-needed-prefixed path for a target locale from the
+// current URL (default locale `de` is unprefixed). Client-only — uses
+// window; called from the save handler.
+function localePath(lang: string): string {
+  const p =
+    window.location.pathname.replace(/^\/(de|fr|it|en)(?=\/|$)/, "") || "/";
+  return lang === "de" ? p : `/${lang}${p === "/" ? "" : p}`;
+}
+
 function Field({
   label,
   required,

src/lib/auth.ts

@@ -111,6 +111,13 @@ export const authConfig: NextAuthConfig = {
         if (typeof profile.sub === "string") {
           token.sub = profile.sub;
         }
+        // Capture the user's preferred language (OIDC `locale` claim,
+        // mapped from ZITADEL preferredLanguage). Read once at sign-in;
+        // middleware uses it to land the user on their language a
+        // single time per login. Stored as-is and validated downstream.
+        if (typeof claims.locale === "string") {
+          token.locale = claims.locale;
+        }
       }
       return token;
     },
@@ -140,6 +147,7 @@ export const authConfig: NextAuthConfig = {
         // both legacy " (Personal)" suffix and current "personal-{8hex}"
         // opaque names.
         isPersonal: isPersonalOrgName(orgName),
+        locale: (token.locale as string | undefined) ?? undefined,
       };
       (session as any).platformUser = sessionUser;
       // Also overwrite session.user so any client-side code that uses

src/lib/zitadel.ts

@@ -569,6 +569,7 @@ export async function updateHumanUserProfile(params: {
   userId: string;
   givenName: string;
   familyName: string;
+  preferredLanguage?: string;
 }): Promise<UpdateHumanUserProfileResult> {
   const path = `/v2/users/human/${encodeURIComponent(params.userId)}`;
   // Compose the displayName ourselves so ZITADEL stores something
@@ -579,13 +580,22 @@ export async function updateHumanUserProfile(params: {
   type ZitadelUpdateResponse = {
     details?: { changeDate?: string };
   };
-  await zitadelFetch<ZitadelUpdateResponse>(path, "PUT", {
+  // preferredLanguage is part of the same `profile` block; include it
-    profile: {
+  // only when provided so a name-only update doesn't clobber it.
+  const profile: {
+    givenName: string;
+    familyName: string;
+    displayName: string;
+    preferredLanguage?: string;
+  } = {
     givenName: params.givenName,
     familyName: params.familyName,
     displayName,
-    },
+  };
-  });
+  if (params.preferredLanguage) {
+    profile.preferredLanguage = params.preferredLanguage;
+  }
+  await zitadelFetch<ZitadelUpdateResponse>(path, "PUT", { profile });
   // Re-fetch the user to read back the canonical displayName ZITADEL
   // committed. Should match what we sent, but reading from the source
   // of truth catches any sanitization ZITADEL might apply.
@@ -607,6 +617,8 @@ export interface HumanUserDetail {
   familyName: string;
   displayName: string;
   email: string;
+  /** ZITADEL profile preferredLanguage (e.g. "de"); "" if unset. */
+  preferredLanguage: string;
 }
 
 export async function getHumanUserDetail(
@@ -620,6 +632,7 @@ export async function getHumanUserDetail(
           givenName?: string;
           familyName?: string;
           displayName?: string;
+          preferredLanguage?: string;
         };
         email?: { email?: string };
       };
@@ -636,5 +649,6 @@ export async function getHumanUserDetail(
     familyName: human?.profile?.familyName ?? "",
     displayName: human?.profile?.displayName ?? "",
     email: human?.email?.email ?? "",
+    preferredLanguage: human?.profile?.preferredLanguage ?? "",
   };
 }

src/messages/de.json

@@ -48,7 +48,8 @@
     "personalCardTitle": "Privat",
     "personalCardDescription": "Für Sie persönlich.",
     "companyCardTitle": "Unternehmen",
-    "companyCardDescription": "Für Ihr Unternehmen oder Team."
+    "companyCardDescription": "Für Ihr Unternehmen oder Team.",
+    "languageLabel": "Sprache"
   },
   "onboarding": {
     "loading": "Status wird geladen…",
@@ -994,7 +995,9 @@
     "saveChanges": "Änderungen speichern",
     "saving": "Speichern…",
     "saved": "Gespeichert.",
-    "missingRequired": "Vor- und Nachname sind erforderlich."
+    "missingRequired": "Vor- und Nachname sind erforderlich.",
+    "languageLabel": "Sprache",
+    "languageHint": "Wird nach der Anmeldung als Ihre Oberflächensprache verwendet."
   },
   "errors": {
     "title": "Etwas ist schiefgelaufen",

src/messages/en.json

@@ -48,7 +48,8 @@
     "personalCardTitle": "Personal",
     "personalCardDescription": "For yourself.",
     "companyCardTitle": "Company",
-    "companyCardDescription": "For your business or team."
+    "companyCardDescription": "For your business or team.",
+    "languageLabel": "Language"
   },
   "onboarding": {
     "loading": "Loading status…",
@@ -994,7 +995,9 @@
     "saveChanges": "Save changes",
     "saving": "Saving…",
     "saved": "Saved.",
-    "missingRequired": "First and last name are required."
+    "missingRequired": "First and last name are required.",
+    "languageLabel": "Language",
+    "languageHint": "Used as your interface language after you sign in."
   },
   "errors": {
     "title": "Something went wrong",

src/messages/fr.json

@@ -48,7 +48,8 @@
     "personalCardTitle": "Particulier",
     "personalCardDescription": "Pour vous.",
     "companyCardTitle": "Entreprise",
-    "companyCardDescription": "Pour votre entreprise ou équipe."
+    "companyCardDescription": "Pour votre entreprise ou équipe.",
+    "languageLabel": "Langue"
   },
   "onboarding": {
     "loading": "Chargement du statut…",
@@ -994,7 +995,9 @@
     "saveChanges": "Enregistrer les modifications",
     "saving": "Enregistrement…",
     "saved": "Enregistré.",
-    "missingRequired": "Le prénom et le nom sont obligatoires."
+    "missingRequired": "Le prénom et le nom sont obligatoires.",
+    "languageLabel": "Langue",
+    "languageHint": "Utilisée comme langue d'interface après votre connexion."
   },
   "errors": {
     "title": "Une erreur est survenue",

src/messages/it.json

@@ -48,7 +48,8 @@
     "personalCardTitle": "Privato",
     "personalCardDescription": "Per lei.",
     "companyCardTitle": "Azienda",
-    "companyCardDescription": "Per la sua azienda o team."
+    "companyCardDescription": "Per la sua azienda o team.",
+    "languageLabel": "Lingua"
   },
   "onboarding": {
     "loading": "Caricamento stato…",
@@ -994,7 +995,9 @@
     "saveChanges": "Salvi modifiche",
     "saving": "Salvataggio…",
     "saved": "Salvato.",
-    "missingRequired": "Nome e cognome sono obbligatori."
+    "missingRequired": "Nome e cognome sono obbligatori.",
+    "languageLabel": "Lingua",
+    "languageHint": "Usata come lingua dell'interfaccia dopo l'accesso."
   },
   "errors": {
     "title": "Si è verificato un errore",

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);

src/types/index.ts

@@ -3,6 +3,8 @@ export interface ZitadelClaims {
   "urn:zitadel:iam:user:resourceowner:id": string;
   "urn:zitadel:iam:user:resourceowner:name": string;
   "urn:zitadel:iam:org:project:roles"?: Record<string, Record<string, string>>;
+  /** Standard OIDC claim; ZITADEL maps the user's preferredLanguage. */
+  locale?: string;
 }
 
 /**
@@ -64,6 +66,14 @@ export interface SessionUser {
    *     user's display name instead (Bug 9 — the org name is opaque).
    */
   isPersonal: boolean;
+  /**
+   * The user's preferred UI language, sourced from the ZITADEL profile
+   * (`preferredLanguage`) via the OIDC `locale` claim at sign-in. Used
+   * once after login to land the user on their language; the header
+   * switcher is a per-session URL override that does not change this.
+   * Undefined for users whose ZITADEL profile predates the claim.
+   */
+  locale?: string;
 }
 
 // PiecedTenant CR (pieced.ch/v1alpha1)