ui/src/lib/app.svelte.ts

@@ -8,6 +8,7 @@ import { gateway } from './gateway';
 import { GatewayError } from './client';
 import { navigate, router } from './router.svelte';
 import { errorKey, localeFrom, setLocale, t, type Locale } from './i18n/index.svelte';
+import { languageNeedsServerSync } from './language';
 import { applyReduceMotion, applyTelegramTheme, applyTheme, type ThemePref } from './theme';
 import {
   insideTelegram,
@@ -455,6 +456,13 @@ async function adoptSession(s: Session): Promise<void> {
     // account here. preferred_language stays the user's saved choice (written from Settings,
     // and used for out-of-app push routing), but the Telegram bot a user signs in through must
     // not dictate the UI: a ru-bot launch on an English system stays English.
+    //
+    // But the banner and out-of-app push routing ARE resolved from preferred_language, so an
+    // explicit device choice the account has not recorded yet (picked while a guest, or
+    // differing from the Telegram system-language seed) would otherwise leave them in the wrong
+    // language until the next Settings change. Reconcile the account to the saved local choice
+    // here; persistLanguageToServer no-ops for guests and when already equal.
+    if (app.localeLocked) void persistLanguageToServer(app.locale);
   } catch (err) {
     handleError(err);
   }
@@ -475,6 +483,9 @@ export async function applyLinkResult(r: LinkResult): Promise<void> {
     return;
   }
   app.profile = await gateway.profileGet();
+  // A guest who chose a language and then linked in place now has a durable account: push the
+  // saved choice so the banner + push routing follow it (see adoptSession).
+  if (app.localeLocked) void persistLanguageToServer(app.locale);
 }
 
 /**
@@ -756,7 +767,7 @@ export function setLocalePref(locale: Locale): void {
  */
 async function persistLanguageToServer(locale: Locale): Promise<void> {
   const p = app.profile;
-  if (!p || p.isGuest || p.preferredLanguage === locale) return;
+  if (!p || !languageNeedsServerSync(p, locale)) return;
   try {
     app.profile = await gateway.profileUpdate({
       displayName: p.displayName,

ui/src/lib/language.test.ts

@@ -0,0 +1,27 @@
+import { describe, it, expect } from 'vitest';
+
+import { languageNeedsServerSync } from './language';
+import type { Profile } from './model';
+
+// The reconciler only reads isGuest + preferredLanguage; a partial cast keeps the fixture small.
+const profile = (over: Partial<Profile>): Profile => ({ isGuest: false, preferredLanguage: 'en', ...over }) as Profile;
+
+describe('languageNeedsServerSync', () => {
+  it('is false without a profile', () => {
+    expect(languageNeedsServerSync(null, 'ru')).toBe(false);
+    expect(languageNeedsServerSync(undefined, 'ru')).toBe(false);
+  });
+
+  it('is false for a guest — guests keep only the client preference', () => {
+    expect(languageNeedsServerSync(profile({ isGuest: true, preferredLanguage: 'en' }), 'ru')).toBe(false);
+  });
+
+  it('is false when the account already matches the locale', () => {
+    expect(languageNeedsServerSync(profile({ preferredLanguage: 'ru' }), 'ru')).toBe(false);
+  });
+
+  it('is true for a real account whose stored language differs (banner + push follow it)', () => {
+    expect(languageNeedsServerSync(profile({ preferredLanguage: 'en' }), 'ru')).toBe(true);
+    expect(languageNeedsServerSync(profile({ preferredLanguage: 'ru' }), 'en')).toBe(true);
+  });
+});

ui/src/lib/language.ts

@@ -0,0 +1,22 @@
+// Interface-language reconciliation. Kept out of app.svelte.ts (a runes module that the
+// node-env Vitest layer cannot import) so the decision is unit-testable.
+
+import type { Locale } from './i18n/catalog';
+import type { Profile } from './model';
+
+/**
+ * languageNeedsServerSync reports whether the durable account's `preferred_language` should be
+ * rewritten to the chosen interface `locale`. It is true only for a real (non-guest) account
+ * whose stored language differs from the locale; guests keep only the client-side preference,
+ * and an already-matching account is a no-op.
+ *
+ * The UI language follows the device (the local choice / system guess), but the advertising
+ * banner and out-of-app push routing are resolved server-side from `preferred_language`. A saved
+ * device choice the account has not yet recorded — picked while a guest, or differing from the
+ * Telegram system-language seed — would otherwise leave the banner and pushes in the wrong
+ * language until the next Settings change. Both the Settings control and the on-load reconciler
+ * gate their write on this.
+ */
+export function languageNeedsServerSync(profile: Profile | null | undefined, locale: Locale): boolean {
+  return !!profile && !profile.isGuest && profile.preferredLanguage !== locale;
+}