diff --git a/lib/__tests__/plugin-i18n.test.ts b/lib/__tests__/plugin-i18n.test.ts
new file mode 100644
index 000000000..eced2762a
--- /dev/null
+++ b/lib/__tests__/plugin-i18n.test.ts
@@ -0,0 +1,32 @@
+import { describe, it, expect } from 'vitest';
+import { lookup, interpolate, localeChain } from '../plugin-sandbox/plugin-i18n';
+
+const tables = {
+  en: { hello: 'Hello {name}', only_en: 'English only' },
+  de: { hello: 'Hallo {name}' },
+  'zh-TW': { hello: '你好 {name}' },
+};
+
+describe('plugin translations', () => {
+  it('falls back from a regional locale to its language', () => {
+    // de-CH used to skip straight to English.
+    expect(lookup(tables, 'de-CH', 'hello')).toBe('Hallo {name}');
+  });
+
+  it('matches locale names regardless of case', () => {
+    expect(lookup(tables, 'zh-tw', 'hello')).toBe('你好 {name}');
+  });
+
+  it('falls back to English, then reports a missing key', () => {
+    expect(lookup(tables, 'de', 'only_en')).toBe('English only');
+    expect(lookup(tables, 'de', 'missing')).toBeUndefined();
+  });
+
+  it('builds the chain most specific first', () => {
+    expect(localeChain('pt-BR')).toEqual(['pt-BR', 'pt', 'en']);
+  });
+
+  it('fills placeholders', () => {
+    expect(interpolate('Hallo {name}', { name: 'Ana' })).toBe('Hallo Ana');
+  });
+});
diff --git a/lib/__tests__/plugin-sandbox-loader.test.ts b/lib/__tests__/plugin-sandbox-loader.test.ts
index 1df200de3..70e0b3b03 100644
--- a/lib/__tests__/plugin-sandbox-loader.test.ts
+++ b/lib/__tests__/plugin-sandbox-loader.test.ts
@@ -8,6 +8,7 @@ const mocks = vi.hoisted(() => ({
   downloadManagedBundle: vi.fn(),
   createBackgroundInstance: vi.fn(),
   registerShortcuts: vi.fn((..._args: unknown[]) => () => {}),
+  setLocale: vi.fn(),
 }));
 
 vi.mock('../plugin-storage', () => ({
@@ -22,6 +23,7 @@ vi.mock('../plugin-sandbox/bundle-fetch', () => ({
 }));
 vi.mock('../plugin-sandbox/host-bridge', () => ({
   createBackgroundInstance: (...args: unknown[]) => mocks.createBackgroundInstance(...args),
+  forEachInstance: (_id: string | null, fn: (i: { setLocale: (l: string) => void }) => void) => fn({ setLocale: mocks.setLocale }),
   SandboxInstance: class {},
 }));
 vi.mock('../plugin-sandbox/host-api', () => ({
@@ -31,7 +33,7 @@ vi.mock('../plugin-sandbox/shortcuts', () => ({
   registerShortcuts: (...args: unknown[]) => mocks.registerShortcuts(...args),
 }));
 
-import { loadSandboxedPlugin, unloadSandboxedPlugin, setSandboxStoreAccessor } from '../plugin-sandbox/loader';
+import { loadSandboxedPlugin, unloadSandboxedPlugin, setSandboxStoreAccessor, setSandboxLocale } from '../plugin-sandbox/loader';
 import { get as getActive } from '../plugin-sandbox/registry';
 
 const CODE = 'export default { activate() {} }';
@@ -205,3 +207,15 @@ describe('plugin shortcuts', () => {
     expect(mocks.registerShortcuts).toHaveBeenCalledWith(expect.anything(), SHORTCUT);
   });
 });
+
+describe('setSandboxLocale', () => {
+  it('switches running plugins to the new language', () => {
+    // Running plugins used to keep the old language until a reload.
+    setSandboxLocale('en');
+    mocks.setLocale.mockClear();
+
+    setSandboxLocale('de');
+
+    expect(mocks.setLocale).toHaveBeenCalledWith('de');
+  });
+});
diff --git a/lib/plugin-sandbox/loader.ts b/lib/plugin-sandbox/loader.ts
index 679661ae4..d7ecf6fe0 100644
--- a/lib/plugin-sandbox/loader.ts
+++ b/lib/plugin-sandbox/loader.ts
@@ -16,7 +16,7 @@ import {
 import { useMessageListTabsStore } from '@/stores/message-list-tabs-store';
 import { verifyBundle } from './bundle-integrity';
 import { downloadManagedBundle } from './bundle-fetch';
-import { createBackgroundInstance } from './host-bridge';
+import { createBackgroundInstance, forEachInstance } from './host-bridge';
 import { resolvePluginTier } from './tier';
 import { register as registerActive, deregister as deregisterActive, all as allActiveEntries } from './registry';
 import { cancelPluginDialogs } from './host-api';
@@ -51,12 +51,17 @@ export function setSandboxLocale(locale: string): void {
   // good locale back to '' - the initial 'en' default stands until the real
   // locale arrives via the store subscription.
   if (!locale) return;
+  const changed = locale !== currentLocale;
   currentLocale = locale;
   // Background instances read `currentLocale` at load time; the slot-iframe
   // component reads this global at spawn time (plugin-iframe-slot.tsx). Keep
-  // both in step from one place. Already-running instances are not re-pushed,
-  // so a locale switch only affects plugins/slots loaded afterwards.
+  // both in step from one place.
   (globalThis as unknown as { __APP_LOCALE__?: string }).__APP_LOCALE__ = locale;
+  if (!changed) return;
+  // Running plugins switch too: their slots re-render in the new language
+  // and their onLocaleChange handlers run.
+  forEachInstance(null, (instance) => instance.setLocale(locale));
+  void themeHooks.onLocaleChange.emit({ locale });
 }
 
 // ─── Bundle fetch ─────────────────────────────────────────────
diff --git a/lib/plugin-sandbox/plugin-i18n.ts b/lib/plugin-sandbox/plugin-i18n.ts
new file mode 100644
index 000000000..52d84283c
--- /dev/null
+++ b/lib/plugin-sandbox/plugin-i18n.ts
@@ -0,0 +1,40 @@
+// Translation lookup for plugin strings (api.i18n.t). Pure, so the runtime
+// inside the sandbox and the tests share it.
+
+export type LocaleTables = Record<string, Record<string, string>>;
+
+/**
+ * Locales to try for `locale`, most specific first: `de-CH` → `de-CH`, `de`,
+ * then `en`. Matching ignores case (`zh-tw` finds `zh-TW`).
+ */
+export function localeChain(locale: string): string[] {
+  const chain: string[] = [];
+  const parts = (locale || 'en').split('-');
+  for (let i = parts.length; i > 0; i--) chain.push(parts.slice(0, i).join('-'));
+  if (!chain.includes('en')) chain.push('en');
+  return chain;
+}
+
+function findTable(tables: LocaleTables, locale: string): Record<string, string> | undefined {
+  if (tables[locale]) return tables[locale];
+  const lower = locale.toLowerCase();
+  const key = Object.keys(tables).find((k) => k.toLowerCase() === lower);
+  return key ? tables[key] : undefined;
+}
+
+/** The text for `key`, or undefined when no table has it. */
+export function lookup(tables: LocaleTables, locale: string, key: string): string | undefined {
+  for (const loc of localeChain(locale)) {
+    const value = findTable(tables, loc)?.[key];
+    if (typeof value === 'string') return value;
+  }
+  return undefined;
+}
+
+/** Replace `{name}` placeholders. */
+export function interpolate(text: string, vars?: Record<string, string | number>): string {
+  if (!vars) return text;
+  let out = text;
+  for (const [k, v] of Object.entries(vars)) out = out.split('{' + k + '}').join(String(v));
+  return out;
+}
diff --git a/lib/plugin-sandbox/runtime.tsx b/lib/plugin-sandbox/runtime.tsx
index 03970b477..93982f340 100644
--- a/lib/plugin-sandbox/runtime.tsx
+++ b/lib/plugin-sandbox/runtime.tsx
@@ -28,6 +28,7 @@ import type {
   SlotInit,
 } from './protocol';
 import { themeSnapshotToCSS, type ThemeSnapshot } from './host-theme';
+import { interpolate, lookup, type LocaleTables } from './plugin-i18n';
 import type { SlotName } from '../plugin-types';
 import { AddressBook, ContactCard } from '../jmap/types';
 import { EncryptionAtRestConfig, PublicKeyInput } from '@/stores/account-security-store';
@@ -66,6 +67,19 @@ const pendingApi = new Map<string, { resolve: (v: unknown) => void; reject: (err
 // The plugin's settings. One object for the iframe's lifetime, updated in
 // place on 'settings-change', so code that kept a reference sees new values.
 const settingsState: Record<string, unknown> = {};
+
+// The app language, pushed by the host on init and on every change.
+const localeListeners = new Set<(locale: string) => void>();
+function currentLocale(): string {
+  return (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ || 'en';
+}
+function applyLocale(locale: string): void {
+  (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ = locale;
+  document.documentElement.lang = locale;
+  for (const listener of localeListeners) {
+    try { listener(locale); } catch (err) { console.error('[plugin] locale listener failed', err); }
+  }
+}
 const settingsListeners = new Set<(settings: Record<string, unknown>) => void>();
 
 function applySettings(next: Record<string, unknown>): void {
@@ -229,6 +243,7 @@ interface PluginKeywordCounts {
 
 function buildPluginApi(manifest: PluginManifest) {
   applySettings({ ...manifest.settings });
+  const localeTables: LocaleTables = JSON.parse(JSON.stringify(manifest.locales ?? {}));
   return {
     plugin: {
       id: manifest.id,
@@ -495,22 +510,27 @@ function buildPluginApi(manifest: PluginManifest) {
     },
     // Localization for plugins. The host pushes the active locale (init +
     // 'locale-change'); `t` resolves a key against the plugin's declared
-    // `locales` map (manifest.locales), falling back to English then the key
-    // itself, with optional {placeholder} interpolation.
+    // `locales` map (manifest.locales) plus anything added at runtime,
+    // trying `de-CH`, then `de`, then English, then the key itself, with
+    // optional {placeholder} interpolation.
     i18n: {
       get locale(): string {
-        return (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ || 'en';
+        return currentLocale();
       },
+      getLocale: () => currentLocale(),
       t(key: string, vars?: Record<string, string | number>): string {
-        const loc = (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ || 'en';
-        const tables = manifest.locales || {};
-        let out = tables[loc]?.[key] ?? tables['en']?.[key] ?? key;
-        if (vars) {
-          for (const [k, v] of Object.entries(vars)) {
-            out = out.split('{' + k + '}').join(String(v));
-          }
-        }
-        return out;
+        return interpolate(lookup(localeTables, currentLocale(), key) ?? key, vars);
+      },
+      /** Whether any table has `key`, for `has(k) ? t(k) : fallback`. */
+      has: (key: string) => lookup(localeTables, currentLocale(), key) !== undefined,
+      /** Add or override strings for one locale. */
+      addTranslations(locale: string, strings: Record<string, string>): void {
+        localeTables[locale] = { ...(localeTables[locale] ?? {}), ...strings };
+      },
+      /** Called with the new locale when the user switches language. Returns an unsubscribe function. */
+      onLocaleChange: (listener: (locale: string) => void) => {
+        localeListeners.add(listener);
+        return () => { localeListeners.delete(listener); };
       },
     },
   };
@@ -664,7 +684,11 @@ function bootSlot(payload: SlotInit): void {
     React.useEffect(() => {
       const bump = () => setSettingsVersion((v) => v + 1);
       settingsListeners.add(bump);
-      return () => { settingsListeners.delete(bump); };
+      localeListeners.add(bump);
+      return () => {
+        settingsListeners.delete(bump);
+        localeListeners.delete(bump);
+      };
     }, []);
     React.useEffect(() => {
       if (!wrapRef.current) return;
@@ -698,7 +722,7 @@ async function handleInit(payload: InitPayload): Promise<void> {
   mode = payload.mode;
   // Make the active locale available to plugin code (api.i18n) right away -
   // not only after the first 'locale-change' push.
-  (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ = payload.locale;
+  applyLocale(payload.locale);
   try {
     if (payload.mode === 'background') {
       await bootBackground(payload);
@@ -780,7 +804,7 @@ function handleHostMessage(ev: MessageEvent): void {
     }
 
     case 'locale-change':
-      (globalThis as unknown as { __PLUGIN_LOCALE__?: string }).__PLUGIN_LOCALE__ = msg.locale;
+      applyLocale(msg.locale);
       break;
 
     case 'theme-change':