From d3d27dc08d40959fc0685c4979c4da67a643257d Mon Sep 17 00:00:00 2001
From: Edward <43848523+thomashacker@users.noreply.github.com>
Date: Tue, 6 Oct 2026 17:00:21 +0200
Subject: [PATCH] [client] Only offer locales listed in _index.json (#8091)

---
 client/ui/frontend/src/lib/i18n.ts | 23 +++++++++++++++++++----
 1 file changed, 19 insertions(+), 4 deletions(-)

diff --git a/client/ui/frontend/src/lib/i18n.ts b/client/ui/frontend/src/lib/i18n.ts
index 87cc996dd..25f2b282a 100644
--- a/client/ui/frontend/src/lib/i18n.ts
+++ b/client/ui/frontend/src/lib/i18n.ts
@@ -40,14 +40,29 @@ function detectBrowserLanguage(available: string[]): string | null {
     return null;
 }
 
-// An empty persisted language code is the Go-side signal for first run.
+// Only locales listed in _index.json are shipped. A bundle on disk without an
+// _index.json row (e.g. a new Crowdin language not released yet) must not be
+// auto-detected or loaded, matching the Go side, which rejects saving it.
+async function shippedLanguages(): Promise<string[]> {
+    const onDisk = Object.keys(resources);
+    try {
+        const listed = new Set<string>((await I18n.Languages()).map((l) => l.code));
+        return onDisk.filter((code) => listed.has(code));
+    } catch (e) {
+        console.warn("load shipped languages failed, using all bundled locales", e);
+        return onDisk;
+    }
+}
+
+// An empty persisted language code is the Go-side signal for first run. A
+// persisted code that is no longer shipped is treated the same way.
 export async function initI18n(): Promise<void> {
-    const available = Object.keys(resources);
+    const available = await shippedLanguages();
     let language = "en";
     let firstRun = false;
     try {
         const prefs = await Preferences.Get();
-        if (prefs?.language) {
+        if (prefs?.language && available.includes(prefs.language)) {
             language = prefs.language;
         } else {
             firstRun = true;
@@ -68,7 +83,7 @@ export async function initI18n(): Promise<void> {
         fallbackLng: "en",
         defaultNS: "common",
         ns: ["common"],
-        resources,
+        resources: Object.fromEntries(available.map((code) => [code, resources[code]])),
         interpolation: {
             prefix: "{",
             suffix: "}",