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

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", {
-    profile: {
-      givenName: params.givenName,
-      familyName: params.familyName,
-      displayName,
-    },
-  });
+  // preferredLanguage is part of the same `profile` block; include it
+  // 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 ?? "",
   };
 }