Seed identity locale from the URL when null

Signed-in viewers with no saved preference get the current
URL locale once so later unprefixed visits have a default.

Signed-off-by: Émile Ré <emile@probo.com>

apps/compliance-portal/src/lib/i18n/useEnsureViewerLocale.ts

@@ -0,0 +1,55 @@
+// Copyright (c) 2026 Probo Inc <hello@probo.com>.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+import { useEffect, useRef } from "react";
+import { graphql, useFragment } from "react-relay";
+
+import type { useEnsureViewerLocale_identity$key } from "./__generated__/useEnsureViewerLocale_identity.graphql";
+import { useLocale } from "./useLocale";
+import { useUpdateLocale } from "./useUpdateLocale";
+
+const ensureViewerLocaleFragment = graphql`
+  fragment useEnsureViewerLocale_identity on Identity {
+    locale
+  }
+`;
+
+// Seeds Identity.locale once when it is still null, using the current URL
+// locale. Never overwrites an existing preference (shared-link safe).
+export function useEnsureViewerLocale(
+  identityKey: useEnsureViewerLocale_identity$key | null,
+) {
+  const identity = useFragment(ensureViewerLocaleFragment, identityKey);
+  const urlLocale = useLocale();
+  const [updateLocale] = useUpdateLocale();
+  const firedRef = useRef(false);
+
+  useEffect(() => {
+    if (identity == null || identity.locale != null || firedRef.current) {
+      return;
+    }
+
+    firedRef.current = true;
+    void updateLocale(urlLocale, { errorToast: false }).catch(() => {
+      // Allow a later mount / navigation to retry if the seed failed.
+      firedRef.current = false;
+    });
+  }, [identity, updateLocale, urlLocale]);
+}

apps/compliance-portal/src/lib/i18n/useUpdateLocale.ts

@@ -18,6 +18,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+import type { MutationFeedback } from "@probo/relay";
 import { useCallback } from "react";
 import { useTranslation } from "react-i18next";
 import { graphql } from "react-relay";
@@ -45,10 +46,16 @@ export function useUpdateLocale() {
     errorToast: t("locale.updateFailed"),
   });
 
-  const updateLocale = useCallback(async (locale: UrlLocale) => {
-    await commit({
-      variables: { input: { locale } },
-    });
+  const updateLocale = useCallback(async (
+    locale: UrlLocale,
+    feedback?: MutationFeedback,
+  ) => {
+    await commit(
+      {
+        variables: { input: { locale } },
+      },
+      feedback,
+    );
   }, [commit]);
 
   return [updateLocale, isUpdating] as const;

apps/compliance-portal/src/pages/MainLayout.tsx

@@ -27,6 +27,7 @@ import { LocaleMismatchCallout } from "#/components/LocaleMismatchCallout";
 import { PoweredBy } from "#/components/PoweredBy/PoweredBy";
 import { TopBar } from "#/components/TopBar/TopBar";
 import { useResumeAccessRequest } from "#/lib/auth/useResumeAccessRequest";
+import { useEnsureViewerLocale } from "#/lib/i18n/useEnsureViewerLocale";
 import { SubscribeDialogProvider } from "#/lib/mailingList/SubscribeDialogProvider";
 
 import type { MainLayoutQuery } from "./__generated__/MainLayoutQuery.graphql";
@@ -36,6 +37,7 @@ export const mainLayoutQuery = graphql`
     viewer {
       __typename
       ...LocaleMismatchCallout_identity
+      ...useEnsureViewerLocale_identity
     }
     ...TopBar_query
     ...SubscribeDialogProvider_query
@@ -53,6 +55,8 @@ export function MainLayout({ queryRef }: MainLayoutProps) {
 
   // Resume a deferred "request access" once the user lands back authenticated.
   useResumeAccessRequest(data.viewer != null);
+  // First signed-in visit: seed Identity.locale from the URL when still null.
+  useEnsureViewerLocale(data.viewer ?? null);
 
   // Document viewer fills the viewport under the TopBar and scrolls its own
   // stage; every other page uses normal document flow so the footer sits after