diff --git a/apps/desktop/src/main/__tests__/attachment-preflight.test.ts b/apps/desktop/src/main/__tests__/attachment-preflight.test.ts
index d38c2c008c..f7b7aee817 100644
--- a/apps/desktop/src/main/__tests__/attachment-preflight.test.ts
+++ b/apps/desktop/src/main/__tests__/attachment-preflight.test.ts
@@ -29,19 +29,19 @@ describe('attachment preflight (before session create)', () => {
       size: 100,
       source: { type: 'file' as const, file: { size: 100 } },
     }));
-    assert.throws(() => preflightAttachmentItems(items), /8/);
+    assert.throws(() => preflightAttachmentItems(items, 'zh'), /8/);
   });
 
   test('rejects an oversized File so no empty session is created', () => {
     assert.throws(
-      () => preflightAttachmentItems([{ size: CAP + 1, source: { type: 'file', file: { size: CAP + 1 } } }]),
+      () => preflightAttachmentItems([{ size: CAP + 1, source: { type: 'file', file: { size: CAP + 1 } } }], 'zh'),
       /50MB/,
     );
   });
 
   test('rejects an oversized approval-token attachment by pending size', () => {
     assert.throws(
-      () => preflightAttachmentItems([{ size: CAP + 1, source: { type: 'approval', approvalId: 'a1' } }]),
+      () => preflightAttachmentItems([{ size: CAP + 1, source: { type: 'approval', approvalId: 'a1' } }], 'zh'),
       /50MB/,
     );
   });
@@ -52,9 +52,17 @@ describe('attachment preflight (before session create)', () => {
         preflightAttachmentItems([
           { size: 10, source: { type: 'approval', approvalId: 'dup' } },
           { size: 10, source: { type: 'approval', approvalId: 'dup' } },
-        ]),
+        ], 'zh'),
       /重复/,
     );
+    assert.throws(
+      () =>
+        preflightAttachmentItems([
+          { size: 10, source: { type: 'approval', approvalId: 'dup' } },
+          { size: 10, source: { type: 'approval', approvalId: 'dup' } },
+        ], 'en'),
+      /already added/,
+    );
   });
 
   test('passes approval tokens and files under the cap', () => {
@@ -62,7 +70,7 @@ describe('attachment preflight (before session create)', () => {
       preflightAttachmentItems([
         { size: 100, source: { type: 'approval', approvalId: 'a1' } },
         { size: 100, source: { type: 'file', file: { size: 100 } } },
-      ]),
+      ], 'zh'),
     );
   });
 });
\ No newline at end of file
diff --git a/apps/desktop/src/main/__tests__/conversation-markdown.test.ts b/apps/desktop/src/main/__tests__/conversation-markdown.test.ts
index cd8cafacec..7d56f293d2 100644
--- a/apps/desktop/src/main/__tests__/conversation-markdown.test.ts
+++ b/apps/desktop/src/main/__tests__/conversation-markdown.test.ts
@@ -58,10 +58,11 @@ describe('renderConversationMarkdown', () => {
         modelId: 'fake',
       },
     ];
-    const md = renderConversationMarkdown('skill session', messages);
+    const md = renderConversationMarkdown('skill session', messages, 'zh');
     assert.match(md, /## 你/);
     assert.ok(md.includes(typed), 'export shows the typed prompt');
     assert.ok(!md.includes('<invoked-skill'), 'export must not include the skill envelope');
     assert.ok(!md.includes('Secret skill body'), 'export must not include skill body');
+    assert.match(renderConversationMarkdown('skill session', messages, 'en'), /## You/);
   });
 });
diff --git a/apps/desktop/src/main/__tests__/model-catalog-choices.test.ts b/apps/desktop/src/main/__tests__/model-catalog-choices.test.ts
index 382d26e64f..b962bebaad 100644
--- a/apps/desktop/src/main/__tests__/model-catalog-choices.test.ts
+++ b/apps/desktop/src/main/__tests__/model-catalog-choices.test.ts
@@ -186,6 +186,7 @@ describe('model catalog picker helpers', () => {
         }),
       ],
       '',
+      'zh',
     );
     const keys = options.map(([key]) => key);
     assert.ok(
@@ -198,4 +199,9 @@ describe('model catalog picker helpers', () => {
       `unsupported Codex model was offered: ${JSON.stringify(keys)}`,
     );
   });
+
+  it('labels a saved-but-unavailable selection in the UI locale', () => {
+    const [, label] = buildCatalogDailyReviewModelOptions([], 'codex::gone', 'en').at(-1)!;
+    assert.equal(label, 'gone · codex · Currently unavailable');
+  });
 });
diff --git a/apps/desktop/src/main/__tests__/session-error-presentation.test.ts b/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
index 750d1d3cda..7ec008d9f6 100644
--- a/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
+++ b/apps/desktop/src/main/__tests__/session-error-presentation.test.ts
@@ -26,12 +26,13 @@ import { describeTurnErrorClass } from '../../renderer/session-status-presentati
 
 describe('provider capacity presentation', () => {
   it('uses capacity-specific copy instead of the unknown error fallback', () => {
-    assert.match(describeSessionErrorReason('provider_capacity') ?? '', /满载/);
-    assert.match(describeTurnErrorClass('provider_capacity'), /满载/);
+    assert.match(describeSessionErrorReason('provider_capacity', 'zh') ?? '', /满载/);
+    assert.match(describeSessionErrorReason('provider_capacity', 'en') ?? '', /at capacity/);
+    assert.match(describeTurnErrorClass('provider_capacity', 'zh'), /满载/);
   });
 
   it('does not recommend an immediate direct retry', () => {
-    const label = describeTurnErrorClass('provider_capacity');
+    const label = describeTurnErrorClass('provider_capacity', 'zh');
     assert.match(label, /等几分钟|换一个模型/);
     assert.doesNotMatch(label, /直接重试/);
   });
diff --git a/apps/desktop/src/renderer/attachment-preflight.ts b/apps/desktop/src/renderer/attachment-preflight.ts
index 0af114c7a9..33891c2fdc 100644
--- a/apps/desktop/src/renderer/attachment-preflight.ts
+++ b/apps/desktop/src/renderer/attachment-preflight.ts
@@ -38,7 +38,7 @@ type PreflightItem = {
  * File blobs are sized by the browser File object; approval-token attachments
  * are sized by the pending size stamped at pick time (main re-stats).
  */
-export function preflightAttachmentItems(items: readonly PreflightItem[], locale: UiLocale = 'zh'): void {
+export function preflightAttachmentItems(items: readonly PreflightItem[], locale: UiLocale): void {
   const copy = getDesktopConversationCopy(locale).attachments;
   if (items.length > MAX_ATTACHMENT_COUNT) throw new Error(copy.tooMany);
   const seen = new Set<string>();
diff --git a/apps/desktop/src/renderer/conversation-markdown.ts b/apps/desktop/src/renderer/conversation-markdown.ts
index 526f41d701..d89d291fd7 100644
--- a/apps/desktop/src/renderer/conversation-markdown.ts
+++ b/apps/desktop/src/renderer/conversation-markdown.ts
@@ -43,7 +43,7 @@ import { getShellRemainingCopy } from './locales/shell-remaining-copy.js';
  *   export that the user is going to paste somewhere public.
  * - **user text** left untouched (the user typed it, they own it).
  */
-export function renderConversationMarkdown(sessionName: string, messages: StoredMessage[], locale: UiLocale = 'zh'): string {
+export function renderConversationMarkdown(sessionName: string, messages: StoredMessage[], locale: UiLocale): string {
   const copy = getShellRemainingCopy(locale).conversationExport;
   const lines: string[] = [];
   lines.push(`# ${sessionName}`);
diff --git a/apps/desktop/src/renderer/derive-turn-lineage-badges.ts b/apps/desktop/src/renderer/derive-turn-lineage-badges.ts
index f6aa1d8373..c90ab67484 100644
--- a/apps/desktop/src/renderer/derive-turn-lineage-badges.ts
+++ b/apps/desktop/src/renderer/derive-turn-lineage-badges.ts
@@ -46,11 +46,11 @@ export interface TurnLineageBadgeInput {
   regeneratedToTurnId?: string;
   /** True when the target turn id still exists in the materialized view. */
   existsTurn(turnId: string): boolean;
-  locale?: UiLocale;
+  locale: UiLocale;
 }
 
 export function deriveTurnLineageBadges(input: TurnLineageBadgeInput): TurnLineageBadge[] {
-  const copy = getDesktopConversationCopy(input.locale ?? 'zh').lineage;
+  const copy = getDesktopConversationCopy(input.locale).lineage;
   const badges: TurnLineageBadge[] = [];
 
   const forwardFrom = input.regeneratedFromTurnId ?? input.retriedFromTurnId;
diff --git a/apps/desktop/src/renderer/features/connection-settings/provider-panel-shared.ts b/apps/desktop/src/renderer/features/connection-settings/provider-panel-shared.ts
index 92dc3a9231..b0ab78e475 100644
--- a/apps/desktop/src/renderer/features/connection-settings/provider-panel-shared.ts
+++ b/apps/desktop/src/renderer/features/connection-settings/provider-panel-shared.ts
@@ -28,7 +28,7 @@ import { cleanErrorMessage } from '../../application/contracts/connection-error-
 
 export type CredentialPresenceStatus = boolean | 'loading' | 'error';
 
-export function providerPanelActionErrorMessage(error: unknown, locale: UiLocale = 'zh'): string {
+export function providerPanelActionErrorMessage(error: unknown, locale: UiLocale): string {
   const shared = getProviderSettingsCopy(locale).shared;
   // Electron wraps ipcMain.handle rejections as "Error invoking remote method
   // '<channel>': Error: <message>". Classify the original message, not the
@@ -64,7 +64,7 @@ export interface ConnectionTestTroubleshootingCopy {
 export function connectionTestFailureFallback(
   result: ConnectionTestResult,
   copy: ConnectionTestTroubleshootingCopy,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const shared = getProviderSettingsCopy(locale).shared;
   if (result.statusCode === 429) return shared.rateLimit;
@@ -82,7 +82,7 @@ export function connectionTestFailureFallback(
 export function connectionTestFailureMessage(
   result: ConnectionTestResult,
   copy: ConnectionTestTroubleshootingCopy,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const fallback = connectionTestFailureFallback(result, copy, locale);
   if (!result.errorMessage) return fallback;
@@ -91,7 +91,7 @@ export function connectionTestFailureMessage(
     : generalizedErrorMessage(new Error(result.errorMessage), fallback);
 }
 
-export function connectionLastTestMessageDisplay(message: string | undefined, locale: UiLocale = 'zh'): string | undefined {
+export function connectionLastTestMessageDisplay(message: string | undefined, locale: UiLocale): string | undefined {
   if (!message) return undefined;
   const trimmed = message.trim();
   if (!trimmed) return undefined;
@@ -105,6 +105,6 @@ export function connectionLastTestMessageDisplay(message: string | undefined, lo
   return classified || copy.statusUnavailable;
 }
 
-export function categoryLabel(category: ProviderCategory, locale: UiLocale = 'zh'): string {
+export function categoryLabel(category: ProviderCategory, locale: UiLocale): string {
   return getProviderSettingsCopy(locale).shared.categories[category];
 }
diff --git a/apps/desktop/src/renderer/features/session-navigation/model/session-project-grouping.ts b/apps/desktop/src/renderer/features/session-navigation/model/session-project-grouping.ts
index 8fa85cf5a6..146d1bcd4f 100644
--- a/apps/desktop/src/renderer/features/session-navigation/model/session-project-grouping.ts
+++ b/apps/desktop/src/renderer/features/session-navigation/model/session-project-grouping.ts
@@ -28,7 +28,7 @@ const UNGROUPED_KEY = '__ungrouped__';
 export function deriveProjectGroups(
   sessions: ReadonlyArray<SessionSummary>,
   projects: ReadonlyArray<ProjectRecord>,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): SessionHistoryGroup[] {
   const sessionsByProject = new Map<string, SessionSummary[]>();
   const canonicalProjectIds = new Map<string, string>();
diff --git a/apps/desktop/src/renderer/model-catalog-choices.ts b/apps/desktop/src/renderer/model-catalog-choices.ts
index d1d8b09a46..aafd68e3b0 100644
--- a/apps/desktop/src/renderer/model-catalog-choices.ts
+++ b/apps/desktop/src/renderer/model-catalog-choices.ts
@@ -48,7 +48,7 @@ export function buildCatalogRecommendedDefaultModel(providerType: ProviderType):
 export function buildCatalogDailyReviewModelOptions(
   connections: readonly (LlmConnection & HostResolvedConnectionCatalog)[],
   currentModelKey: string,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): Array<readonly [string, string]> {
   const current = parseDailyReviewModelKey(currentModelKey);
   const candidates: Array<{ key: string; label: string; safeSourceLabel: string }> = [];
diff --git a/apps/desktop/src/renderer/model-connection-errors.ts b/apps/desktop/src/renderer/model-connection-errors.ts
index fe3fdf5568..d399fcaa41 100644
--- a/apps/desktop/src/renderer/model-connection-errors.ts
+++ b/apps/desktop/src/renderer/model-connection-errors.ts
@@ -46,7 +46,7 @@ export function noRealConnectionReasonFromEvent(event: Extract<SessionEvent, { t
   ).reason;
 }
 
-export function noRealConnectionSetupDescription(reason: string | undefined, locale: UiLocale = 'zh'): string {
+export function noRealConnectionSetupDescription(reason: string | undefined, locale: UiLocale): string {
   const copy = getDesktopConversationCopy(locale).model;
   return reason && Object.hasOwn(copy.configurationReason, reason)
     ? copy.configurationReason[reason as ChatConfigurationReason]
@@ -55,7 +55,7 @@ export function noRealConnectionSetupDescription(reason: string | undefined, loc
 
 export function sessionEventErrorMessage(
   event: Extract<SessionEvent, { type: 'error' }>,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   if (isNoRealConnectionEvent(event)) {
     return noRealConnectionSetupDescription(noRealConnectionReasonFromEvent(event), locale);
@@ -69,7 +69,7 @@ export function sessionEventErrorMessage(
 export function modelSetupToastCopy(
   reason: string | undefined,
   fallback: string,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): { title: string; description: string } {
   const copy = getDesktopConversationCopy(locale).model;
   if (reason === 'connection_missing') {
diff --git a/apps/desktop/src/renderer/session-error-presentation.ts b/apps/desktop/src/renderer/session-error-presentation.ts
index f3bccf1e7b..b1c5fa85a2 100644
--- a/apps/desktop/src/renderer/session-error-presentation.ts
+++ b/apps/desktop/src/renderer/session-error-presentation.ts
@@ -25,7 +25,7 @@ import { getDesktopConversationCopy } from './locales/conversation-copy.js';
  * runtime. Unknown reasons intentionally return undefined so callers can use
  * their existing safe fallback instead of displaying raw provider text.
  */
-export function describeSessionErrorReason(reason: string | undefined, locale: UiLocale = 'zh'): string | undefined {
+export function describeSessionErrorReason(reason: string | undefined, locale: UiLocale): string | undefined {
   const copy = getDesktopConversationCopy(locale).turnError;
   switch (reason?.toLowerCase()) {
     case 'context_overflow':
diff --git a/apps/desktop/src/renderer/session-status-presentation.ts b/apps/desktop/src/renderer/session-status-presentation.ts
index c039d73e30..36b7993054 100644
--- a/apps/desktop/src/renderer/session-status-presentation.ts
+++ b/apps/desktop/src/renderer/session-status-presentation.ts
@@ -97,7 +97,7 @@ export function normalizeSessionSummaryForDisplay<T extends SessionSummary>(sess
  * the UI; they just fall through to the catch-all until the mapping
  * is extended.
  */
-export function describeTurnErrorClass(errorClass: string | undefined, locale: UiLocale = 'zh'): string {
+export function describeTurnErrorClass(errorClass: string | undefined, locale: UiLocale): string {
   const copy = getDesktopConversationCopy(locale).turnError;
   if (!errorClass) return copy.unknown;
   const reasonDescription = describeSessionErrorReason(errorClass, locale);
@@ -169,7 +169,7 @@ export interface FailedTurnExecutionState {
  */
 export function describeFailedTurnExecutionState(
   state: FailedTurnExecutionState,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string | undefined {
   const copy = getDesktopConversationCopy(locale).turnError.executionState;
   if (state.erroredToolCount > 0) return copy.erroredTool;
diff --git a/apps/desktop/src/renderer/settings/bot-chat-shared.tsx b/apps/desktop/src/renderer/settings/bot-chat-shared.tsx
index 101d069aa7..5ec2b5dee9 100644
--- a/apps/desktop/src/renderer/settings/bot-chat-shared.tsx
+++ b/apps/desktop/src/renderer/settings/bot-chat-shared.tsx
@@ -60,7 +60,7 @@ export const BOT_LABELS: Record<BotProvider, { support: 'runtime' | 'credentials
   slack: { support: 'runtime' },
 };
 
-export function botReadinessCopyForSupport(support: 'runtime' | 'credentials' | 'planned', readiness: BotReadinessState, locale: UiLocale = 'zh') {
+export function botReadinessCopyForSupport(support: 'runtime' | 'credentials' | 'planned', readiness: BotReadinessState, locale: UiLocale) {
   const copy = getBotSettingsCopy(locale);
   if (support === 'planned') return copy.planned;
   return copy.readiness[readiness] ?? copy.readiness.scaffolded;
@@ -98,7 +98,7 @@ export function BotBrandLogo(props: { provider: BotProvider; size?: 'compact' |
 export type BotPendingActionName = 'test' | 'connect' | 'restart' | 'disconnect';
 export type BotPendingAction = { provider: BotProvider; action: BotPendingActionName };
 
-export function botStatusDetail(status: BotStatus, locale: UiLocale = 'zh'): string {
+export function botStatusDetail(status: BotStatus, locale: UiLocale): string {
   const copy = getBotSettingsCopy(locale).status;
   switch (status.reason) {
     case 'disabled': return copy.disabled;
diff --git a/apps/desktop/src/renderer/settings/provider-connection-status.ts b/apps/desktop/src/renderer/settings/provider-connection-status.ts
index b74f2c3bab..99935d6575 100644
--- a/apps/desktop/src/renderer/settings/provider-connection-status.ts
+++ b/apps/desktop/src/renderer/settings/provider-connection-status.ts
@@ -62,7 +62,7 @@ export interface ConnectionChipStatus {
  *   readiness, fixed to credential-only language. Matches the doc warning
  *   at SettingsModal `验证通过 ≠ 运行可用`.
  */
-export function connectionChipStatus(connection: LlmConnection, locale: UiLocale = 'zh'): ConnectionChipStatus | null {
+export function connectionChipStatus(connection: LlmConnection, locale: UiLocale): ConnectionChipStatus | null {
   const copy = getProviderSettingsCopy(locale).shared.connectionStatuses;
   // Ahead of every other branch, including needs_reauth: a retired provider
   // has no sign-in left to return to, so any repair-shaped status here would
diff --git a/apps/desktop/src/renderer/settings/settings-error-copy.ts b/apps/desktop/src/renderer/settings/settings-error-copy.ts
index 5456695179..98413eb0cb 100644
--- a/apps/desktop/src/renderer/settings/settings-error-copy.ts
+++ b/apps/desktop/src/renderer/settings/settings-error-copy.ts
@@ -22,7 +22,7 @@ import { type UiLocale } from '@maka/core/ui-locale';
 import { redactSecrets } from '@maka/ui';
 import { getSettingsSharedCopy } from '../locales/settings-shared-copy.js';
 
-export function settingsActionErrorMessage(error: unknown, locale: UiLocale = 'zh'): string {
+export function settingsActionErrorMessage(error: unknown, locale: UiLocale): string {
   const raw = error instanceof Error
     ? error.message
     : typeof error === 'string'
diff --git a/apps/desktop/src/renderer/turn-footer-actions.ts b/apps/desktop/src/renderer/turn-footer-actions.ts
index 3d60ef76f5..02f874b8e5 100644
--- a/apps/desktop/src/renderer/turn-footer-actions.ts
+++ b/apps/desktop/src/renderer/turn-footer-actions.ts
@@ -99,7 +99,7 @@ export interface TurnFooterContext {
    * / other action types stay clickable.
    */
   pendingActions?: ReadonlySet<TurnFooterActionId>;
-  locale?: UiLocale;
+  locale: UiLocale;
 }
 
 /**
@@ -113,7 +113,7 @@ export interface TurnFooterContext {
  */
 export function deriveTurnFooterActions(input: TurnFooterContext): TurnFooterAction[] {
   const { status, hasContent, alreadyRegenerated, pendingActions, metaSummary } = input;
-  const copyText = getDesktopConversationCopy(input.locale ?? 'zh').footer;
+  const copyText = getDesktopConversationCopy(input.locale).footer;
   const actionLabel = copyText.labels;
   const isPending = (id: TurnFooterActionId) => pendingActions?.has(id) ?? false;
   const PENDING_TOOLTIP = copyText.pending;
diff --git a/packages/core/src/relative-time.ts b/packages/core/src/relative-time.ts
index 5ce6f475bd..d31e1f0f70 100644
--- a/packages/core/src/relative-time.ts
+++ b/packages/core/src/relative-time.ts
@@ -93,7 +93,7 @@ function getAbsoluteFormat(uiLocale: UiLocale): Intl.DateTimeFormat {
  * reading a relative label falls back to and a tooltip shows; `@maka/ui` had
  * its own uncached copy of the same `Intl` options until this became public.
  */
-export function formatAbsoluteTimestamp(ts: number, locale: UiLocale = 'zh'): string {
+export function formatAbsoluteTimestamp(ts: number, locale: UiLocale): string {
   return getAbsoluteFormat(locale).format(new Date(ts));
 }
 
@@ -105,7 +105,7 @@ export function formatAbsoluteTimestamp(ts: number, locale: UiLocale = 'zh'): st
 export function formatRelativeTimestamp(
   ts: number,
   now: number = Date.now(),
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const diffMs = relativeAgeMs(ts, now);
   if (diffMs < JUST_NOW_MS) {
@@ -158,7 +158,7 @@ function getCompactFormats(uiLocale: UiLocale): {
 export function formatCompactTimestamp(
   ts: number,
   now: number = Date.now(),
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const diffMs = relativeAgeMs(ts, now);
   if (diffMs <= RELATIVE_HORIZON_MS) return formatRelativeTimestamp(ts, now, locale);
@@ -178,7 +178,7 @@ export function formatCompactTimestamp(
 export function formatSidebarTimestamp(
   ts: number,
   now: number = Date.now(),
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const diffMs = relativeAgeMs(ts, now);
   if (diffMs < JUST_NOW_MS) return JUST_NOW[locale];
diff --git a/packages/core/src/tool-quiet-preview.ts b/packages/core/src/tool-quiet-preview.ts
index e2adb0d7b6..7d4247e8fe 100644
--- a/packages/core/src/tool-quiet-preview.ts
+++ b/packages/core/src/tool-quiet-preview.ts
@@ -230,7 +230,7 @@ export interface ToolInvocationInput {
  */
 export function formatToolInvocationLine(
   item: ToolInvocationInput,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string | undefined {
   const s = strings(locale);
   const args = asRecord(item.args);
@@ -518,7 +518,7 @@ export interface QuietPreview {
  * Primary list/text fields become the main body; remaining fields (error, ok,
  * truncated, …) are appended so diagnostics cannot vanish.
  */
-export function formatQuietJsonValue(value: unknown, locale: UiLocale = 'zh'): QuietPreview {
+export function formatQuietJsonValue(value: unknown, locale: UiLocale): QuietPreview {
   const s = strings(locale);
   if (value === null || value === undefined) {
     return { body: s.empty };
@@ -677,7 +677,7 @@ function formatArrayAsBody(values: unknown[], locale: UiLocale): string {
 export function formatAsKeyValueLines(
   record: Record<string, unknown>,
   depth = 0,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const s = strings(locale);
   if (depth > 3) return redactSecrets(String(record));
diff --git a/packages/ui/src/artifact-preview-registry.ts b/packages/ui/src/artifact-preview-registry.ts
index def9198076..89158dff5c 100644
--- a/packages/ui/src/artifact-preview-registry.ts
+++ b/packages/ui/src/artifact-preview-registry.ts
@@ -81,7 +81,7 @@ function exceedsImagePayloadCap(base64: string): boolean {
   return base64.length > IMAGE_PAYLOAD_MAX_BASE64_LENGTH;
 }
 
-export function formatPreviewSize(sizeBytes: number | undefined, locale: UiLocale = 'zh'): string {
+export function formatPreviewSize(sizeBytes: number | undefined, locale: UiLocale): string {
   if (sizeBytes === undefined || sizeBytes < 0 || !Number.isFinite(sizeBytes)) return getSharedUiCopy(locale).artifact.unknownSize;
   if (sizeBytes < 1024) return `${sizeBytes} B`;
   if (sizeBytes < 1024 * 1024) return `${(sizeBytes / 1024).toFixed(1)} KB`;
diff --git a/packages/ui/src/chat-model-helpers.ts b/packages/ui/src/chat-model-helpers.ts
index 385dc167da..3edc61911f 100644
--- a/packages/ui/src/chat-model-helpers.ts
+++ b/packages/ui/src/chat-model-helpers.ts
@@ -49,7 +49,7 @@ export type { ChatModelChoice } from '@maka/core/chat-model-choice';
 
 export function modelChoiceDescription(
   choice: Pick<ChatModelChoice, 'description' | 'knowledgeCutoff'>,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string | undefined {
   const description = choice.description?.trim();
   const knowledge = choice.knowledgeCutoff?.trim();
@@ -85,7 +85,7 @@ export interface ModelMenuGroup {
  * account email `connection.name` carries for `claude-subscription` /
  * `openai-codex`.
  */
-export function modelMenuGroups(choices: ChatModelChoice[], locale: UiLocale = 'zh'): ModelMenuGroup[] {
+export function modelMenuGroups(choices: ChatModelChoice[], locale: UiLocale): ModelMenuGroup[] {
   const copy = getSharedUiCopy(locale).providers;
   const localizedLabels: Partial<Record<ProviderType, string>> = {
     'MiniMax-cn': copy.minimaxChina,
diff --git a/packages/ui/src/session-status-presentation.ts b/packages/ui/src/session-status-presentation.ts
index a2f5c5cf9a..9037b7db3a 100644
--- a/packages/ui/src/session-status-presentation.ts
+++ b/packages/ui/src/session-status-presentation.ts
@@ -66,7 +66,7 @@ const STATUS_SEMANTIC: Record<SessionStatus, StatusSemantic | undefined> = {
 
 export function presentSessionStatus(
   status: SessionStatus,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): SessionStatusPresentation {
   const semantic = STATUS_SEMANTIC[status];
   return {
@@ -84,7 +84,7 @@ export function presentSessionStatus(
  */
 export function describeBlockedReason(
   reason: SessionBlockedReason | undefined,
-  locale: UiLocale = 'zh',
+  locale: UiLocale,
 ): string {
   const copy = getConversationCopy(locale).sessions.blockedReason;
   return reason ? copy[reason] : copy.unknown;
diff --git a/packages/ui/stories/model-picker.stories.tsx b/packages/ui/stories/model-picker.stories.tsx
index b8cd7f7fd9..7774685f20 100644
--- a/packages/ui/stories/model-picker.stories.tsx
+++ b/packages/ui/stories/model-picker.stories.tsx
@@ -354,7 +354,7 @@ export const SavingDefaultModel: Story = {
   render: () => (
     <div style={{ width: 260 }}>
       <ModelPicker
-        groups={modelMenuGroups(CHOICES)}
+        groups={modelMenuGroups(CHOICES, useUiLocale())}
         value={modelChoiceValue(CHOICES[4]!.connectionSlug, CHOICES[4]!.model)}
         leadingOption={{ value: '', label: '未设置' }}
         renderProviderMark={providerMark}