fix(onboarding): preserve system language in both experiment cohorts (#31810)

src/main/persistence-onboarding-language-cohort.test.ts

@@ -32,9 +32,9 @@ afterEach(async () => {
 
 describe('persisted onboarding language cohorts', () => {
   it.each([
-    { draw: 0, cohort: 'control', language: 'en' },
-    { draw: 1, cohort: 'treatment', language: 'system' }
-  ])('retains $cohort through settings changes and restart', async ({ draw, cohort, language }) => {
+    { draw: 0, cohort: 'control' },
+    { draw: 1, cohort: 'treatment' }
+  ])('retains $cohort through settings changes and restart', async ({ draw, cohort }) => {
     vi.mocked(crypto.randomInt).mockImplementation(() => draw)
     const store = createStore()
     const telemetry = store.getSettings().telemetry
@@ -42,9 +42,10 @@ describe('persisted onboarding language cohorts', () => {
       throw new Error('Fresh profile has no telemetry settings')
     }
     expect(telemetry.onboardingLanguageCohort).toBe(cohort)
-    expect(store.getSettings().uiLanguage).toBe(language)
+    expect(store.getSettings().uiLanguage).toBe('system')
     store.flush()
     expect(readDataFile()).toHaveProperty('settings.telemetry.onboardingLanguageCohort', cohort)
+    expect(readDataFile()).toHaveProperty('settings.uiLanguage', 'system')
 
     store.updateSettings({ uiLanguage: 'fr' })
     store.updateSettings({ telemetry: { ...telemetry, optedIn: false } })

src/main/persistence/loading-store/loaded-cohort-migrations.test.ts

@@ -12,17 +12,17 @@ afterEach(() => vi.resetAllMocks())
 
 describe('onboarding language experiment', () => {
   it.each([
-    { draw: 0, cohort: 'control', language: 'en' },
-    { draw: 1, cohort: 'treatment', language: 'system' }
+    { draw: 0, cohort: 'control' },
+    { draw: 1, cohort: 'treatment' }
   ])(
-    'assigns and freezes the $cohort experience for a new install',
-    ({ draw, cohort, language }) => {
+    'assigns and freezes the $cohort experience with the system language for a new install',
+    ({ draw, cohort }) => {
       vi.mocked(crypto.randomInt).mockImplementation(() => draw)
       const runtime = { loadNeedsSave: false }
       const migration = new LoadedCohortMigrationOperations(runtime)
       const fresh = migration.migrateTelemetry(getDefaultPersistedState('/test'), false)
       expect(fresh.settings.telemetry?.onboardingLanguageCohort).toBe(cohort)
-      expect(fresh.settings.uiLanguage).toBe(language)
+      expect(fresh.settings.uiLanguage).toBe('system')
       expect(runtime.loadNeedsSave).toBe(true)
       const reopened = migration.migrateTelemetry(JSON.parse(JSON.stringify(fresh)), true)
       expect(reopened).toEqual(fresh)

src/main/persistence/loading-store/loaded-cohort-migrations.ts

@@ -96,7 +96,6 @@ export class LoadedCohortMigrationOperations {
       ...state,
       settings: {
         ...state.settings,
-        ...(onboardingLanguageCohort === 'control' ? { uiLanguage: 'en' as const } : {}),
         telemetry: {
           ...existing,
           ...(onboardingLanguageCohort ? { onboardingLanguageCohort } : {}),