diff --git a/docs/settings-reference.md b/docs/settings-reference.md
index 5014cad924..8afcc85427 100644
--- a/docs/settings-reference.md
+++ b/docs/settings-reference.md
@@ -33,7 +33,7 @@ Defaults from `DEFAULT_GLOBAL_SETTINGS`; key scope from `GLOBAL_SETTINGS_KEYS`.
 |---|---|---:|---|
 | `themeMode` | `"dark" \| "light" \| "system"` | `"dark"` | Dashboard theme mode. |
 | `colorTheme` | `ColorTheme` | `"default"` | Dashboard color theme preset. |
-| `language` | `"en" \| "zh-CN" \| "zh-TW" \| "fr" \| "es"` | `undefined` | UI language for the dashboard and TUI. When unset, the dashboard detects from localStorage → browser language and the CLI from `--lang` flag → environment locale, falling back to `en`. Validated at the store write boundary (`validateLocale`); invalid values are dropped. |
+| `language` | `"en" \| "zh-CN" \| "zh-TW" \| "fr" \| "es"` | `undefined` | UI language for the dashboard and TUI. When unset, the dashboard detects from localStorage → browser language and the CLI from `--lang` flag → environment locale, falling back to `en`. Validated at the store write boundary (`validateLocale`); invalid values are dropped. Reset to auto-detect via the dashboard's "Auto" language option or `fn settings set language auto` (clears the persisted key). |
 | `dashboardFontScalePct` | `number` | `100` | Dashboard font scale percentage used by Appearance settings. Valid range: `85` to `125`; applied pre-hydration via document root font-size so board typography (column headers/counts, task cards, and quick-entry text) scales with the setting from first paint. |
 | `defaultProvider` | `string` | `undefined` | Default AI provider. |
 | `defaultModelId` | `string` | `undefined` | Default AI model ID. |
diff --git a/packages/cli/src/commands/__tests__/settings.test.ts b/packages/cli/src/commands/__tests__/settings.test.ts
index a6e8bb1880..1de05dd11e 100644
--- a/packages/cli/src/commands/__tests__/settings.test.ts
+++ b/packages/cli/src/commands/__tests__/settings.test.ts
@@ -156,6 +156,34 @@ describe("settings commands", () => {
     expect(resolveProject).not.toHaveBeenCalled();
   });
 
+  it("runSettingsSet language persists a supported locale globally", async () => {
+    const updateSettings = vi.fn().mockResolvedValue(makeSettings({ language: "zh-TW" } as any));
+    const getSettings = vi.fn().mockResolvedValue(makeSettings({ language: "zh-TW" } as any));
+    (GlobalSettingsStore as unknown as ReturnType<typeof vi.fn>).mockImplementation(() => ({
+      init: vi.fn().mockResolvedValue(undefined),
+      updateSettings,
+      getSettings,
+    }));
+
+    await runSettingsSet("language", "zh-TW");
+
+    expect(updateSettings).toHaveBeenCalledWith({ language: "zh-TW" });
+  });
+
+  it("runSettingsSet language auto clears the persisted locale (null-as-delete)", async () => {
+    const updateSettings = vi.fn().mockResolvedValue(makeSettings({}));
+    const getSettings = vi.fn().mockResolvedValue(makeSettings({}));
+    (GlobalSettingsStore as unknown as ReturnType<typeof vi.fn>).mockImplementation(() => ({
+      init: vi.fn().mockResolvedValue(undefined),
+      updateSettings,
+      getSettings,
+    }));
+
+    await runSettingsSet("language", "auto");
+
+    expect(updateSettings).toHaveBeenCalledWith({ language: null });
+  });
+
   it("runSettingsSet with project updates project-only settings", async () => {
     const updateSettings = vi.fn().mockResolvedValue(makeSettings({ maxConcurrent: 6 }));
     const getSettings = vi.fn().mockResolvedValue(makeSettings({ maxConcurrent: 6 }));
diff --git a/packages/cli/src/commands/settings.ts b/packages/cli/src/commands/settings.ts
index c8bdc331ae..84fea3e418 100644
--- a/packages/cli/src/commands/settings.ts
+++ b/packages/cli/src/commands/settings.ts
@@ -66,7 +66,8 @@ const ENUM_SETTINGS: Record<string, readonly string[]> = {
   worktreeNaming: ["random", "task-id", "task-title"],
   unavailableNodePolicy: ["block", "fallback-local"],
   "worktrunk.onFailure": ["fail", "fallback-native"],
-  language: SUPPORTED_LOCALES,
+  // "auto" clears the persisted locale and reverts to runtime detection.
+  language: [...SUPPORTED_LOCALES, "auto"],
 };
 
 const STRING_SETTINGS: readonly string[] = [
@@ -339,6 +340,16 @@ export async function runSettingsSet(key: string, value: string, projectName?: s
   try {
     const parsedValue = parseValue(validKey, value);
 
+    if (key === "language" && parsedValue === "auto") {
+      // null-as-delete: removes the persisted key so the dashboard re-detects
+      // from the browser and the TUI falls back to the environment locale.
+      await globalStore!.updateSettings({ language: null } as unknown as Partial<GlobalSettings>);
+      console.log();
+      console.log("  ✓ Language reset to auto-detect (browser/environment locale)");
+      console.log();
+      return;
+    }
+
     if (key === "defaultModel") {
       const parts = (parsedValue as string).split("/");
       if (parts.length !== 2) {
diff --git a/packages/cli/src/i18n/__tests__/i18n.test.tsx b/packages/cli/src/i18n/__tests__/i18n.test.tsx
index 05f2f42541..43d86297c3 100644
--- a/packages/cli/src/i18n/__tests__/i18n.test.tsx
+++ b/packages/cli/src/i18n/__tests__/i18n.test.tsx
@@ -2,9 +2,17 @@ import { Text } from "ink";
 import { render } from "ink-testing-library";
 import { createElement } from "react";
 import { I18nextProvider, useTranslation } from "react-i18next";
-import { describe, expect, it } from "vitest";
+import { afterEach, describe, expect, it } from "vitest";
 import { cliI18n, detectEnvLocale, initCliI18n, resolveCliLocale } from "../index.js";
 
+// cliI18n is a module singleton: restore the locale after each test so cases
+// that switch language (zh-CN/fr) can't leak state into later tests.
+afterEach(async () => {
+  if (cliI18n.isInitialized && cliI18n.language !== "en") {
+    await cliI18n.changeLanguage("en");
+  }
+});
+
 describe("detectEnvLocale", () => {
   it("parses POSIX locale env values to a supported locale", () => {
     expect(detectEnvLocale({ LANG: "fr_FR.UTF-8" })).toBe("fr");
diff --git a/packages/core/src/__tests__/settings-precedence.test.ts b/packages/core/src/__tests__/settings-precedence.test.ts
index 8df5c5e82c..f33fcacc67 100644
--- a/packages/core/src/__tests__/settings-precedence.test.ts
+++ b/packages/core/src/__tests__/settings-precedence.test.ts
@@ -38,4 +38,18 @@ describe("settings precedence", () => {
     });
     expect(scoped.project.worktrunk).toEqual({ enabled: false });
   });
+
+  it("validates language at the global write boundary and clears it via null", async () => {
+    // Valid locale persists.
+    await harness.store().updateGlobalSettings({ language: "fr" });
+    expect((await harness.store().getSettings()).language).toBe("fr");
+
+    // Invalid value is dropped at the boundary — prior choice survives.
+    await harness.store().updateGlobalSettings({ language: "klingon" } as never);
+    expect((await harness.store().getSettings()).language).toBe("fr");
+
+    // Explicit null clears the persisted key (reset to runtime auto-detect).
+    await harness.store().updateGlobalSettings({ language: null } as never);
+    expect((await harness.store().getSettings()).language).toBeUndefined();
+  });
 });
diff --git a/packages/core/src/store.ts b/packages/core/src/store.ts
index a7d43b6589..9281ef2de3 100644
--- a/packages/core/src/store.ts
+++ b/packages/core/src/store.ts
@@ -3255,12 +3255,17 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     // Validate the optional UI locale at the write boundary: drop unrecognized
     // values rather than persisting junk into settings.json. Runtime consumers
     // also guard via isLocale, but the contract is `language?: Locale`.
+    // `null` passes through intact — GlobalSettingsStore treats null as
+    // "delete this key", which reverts the language to runtime auto-detect.
     if ("language" in globalPatch) {
-      const validatedLanguage = validateLocale((globalPatch as Record<string, unknown>)["language"]);
-      if (validatedLanguage === undefined) {
-        delete (globalPatch as Record<string, unknown>)["language"];
-      } else {
-        globalPatch.language = validatedLanguage;
+      const rawLanguage = (globalPatch as Record<string, unknown>)["language"];
+      if (rawLanguage !== null) {
+        const validatedLanguage = validateLocale(rawLanguage);
+        if (validatedLanguage === undefined) {
+          delete (globalPatch as Record<string, unknown>)["language"];
+        } else {
+          globalPatch.language = validatedLanguage;
+        }
       }
     }
 
diff --git a/packages/dashboard/app/components/LanguageSelector.tsx b/packages/dashboard/app/components/LanguageSelector.tsx
index e8ca1b68d1..167b9b4e38 100644
--- a/packages/dashboard/app/components/LanguageSelector.tsx
+++ b/packages/dashboard/app/components/LanguageSelector.tsx
@@ -15,7 +15,8 @@ const ENDONYMS: Record<Locale, string> = {
 /** Settings control for choosing the UI language. Applies in place (no reload). */
 export function LanguageSelector() {
   const { t } = useTranslation("app");
-  const { language, supportedLocales, setLanguage } = useLanguage();
+  const { language, supportedLocales, setLanguage, clearLanguage, hasExplicitChoice } =
+    useLanguage();
   const label = t("settings.appearance.language", "Language");
 
   return (
@@ -24,13 +25,22 @@ export function LanguageSelector() {
       {/* role="group" + aria-pressed: toggle-button semantics (radiogroup would
           conflict with aria-pressed and confuse screen readers). */}
       <div className="language-options" role="group" aria-label={label}>
+        <button
+          type="button"
+          className={`language-option${hasExplicitChoice ? "" : " active"}`}
+          onClick={clearLanguage}
+          aria-pressed={!hasExplicitChoice}
+          title={t("settings.appearance.languageAutoHint", "Follow the browser language")}
+        >
+          {t("settings.appearance.languageAuto", "Auto")}
+        </button>
         {supportedLocales.map((locale) => (
           <button
             key={locale}
             type="button"
-            className={`language-option${language === locale ? " active" : ""}`}
+            className={`language-option${hasExplicitChoice && language === locale ? " active" : ""}`}
             onClick={() => setLanguage(locale)}
-            aria-pressed={language === locale}
+            aria-pressed={hasExplicitChoice && language === locale}
             lang={locale}
           >
             {ENDONYMS[locale]}
diff --git a/packages/dashboard/app/components/__tests__/LanguageSelector.test.tsx b/packages/dashboard/app/components/__tests__/LanguageSelector.test.tsx
index 9280935857..58b59a877f 100644
--- a/packages/dashboard/app/components/__tests__/LanguageSelector.test.tsx
+++ b/packages/dashboard/app/components/__tests__/LanguageSelector.test.tsx
@@ -11,14 +11,18 @@ vi.mock("react-i18next", () => ({
 const { LanguageSelector } = await import("../LanguageSelector");
 const mockUseLanguage = vi.mocked(useLanguage);
 const setLanguage = vi.fn();
+const clearLanguage = vi.fn();
 
 describe("LanguageSelector", () => {
   beforeEach(() => {
     setLanguage.mockClear();
+    clearLanguage.mockClear();
     mockUseLanguage.mockReturnValue({
       language: "en",
       supportedLocales: SUPPORTED_LOCALES,
       setLanguage,
+      clearLanguage,
+      hasExplicitChoice: true,
     });
   });
 
@@ -40,4 +44,25 @@ describe("LanguageSelector", () => {
     fireEvent.click(screen.getByText("简体中文"));
     expect(setLanguage).toHaveBeenCalledWith("zh-CN");
   });
+
+  it("offers an Auto option that clears the explicit choice", () => {
+    render(<LanguageSelector />);
+    const auto = screen.getByText("Auto");
+    expect(auto.getAttribute("aria-pressed")).toBe("false");
+    fireEvent.click(auto);
+    expect(clearLanguage).toHaveBeenCalledTimes(1);
+  });
+
+  it("marks Auto as pressed (and no locale) when no explicit choice exists", () => {
+    mockUseLanguage.mockReturnValue({
+      language: "fr", // detected, not chosen
+      supportedLocales: SUPPORTED_LOCALES,
+      setLanguage,
+      clearLanguage,
+      hasExplicitChoice: false,
+    });
+    render(<LanguageSelector />);
+    expect(screen.getByText("Auto").getAttribute("aria-pressed")).toBe("true");
+    expect(screen.getByText("Français").getAttribute("aria-pressed")).toBe("false");
+  });
 });
diff --git a/packages/dashboard/app/hooks/__tests__/useLanguage.test.ts b/packages/dashboard/app/hooks/__tests__/useLanguage.test.ts
index 99101ed096..fbef6512f3 100644
--- a/packages/dashboard/app/hooks/__tests__/useLanguage.test.ts
+++ b/packages/dashboard/app/hooks/__tests__/useLanguage.test.ts
@@ -124,6 +124,29 @@ describe("useLanguage", () => {
     expect(i18nMock.changeLanguage).toHaveBeenCalledWith("zh-TW");
   });
 
+  it("clearLanguage removes the local key, clears the server setting, and re-detects", () => {
+    store[LANGUAGE_STORAGE_KEY] = "fr";
+    vi.stubGlobal("navigator", { languages: ["es-419", "en-US"], language: "es-419" });
+    const { result } = renderHook(() => useLanguage());
+    expect(result.current.hasExplicitChoice).toBe(true);
+
+    act(() => result.current.clearLanguage());
+
+    expect(store[LANGUAGE_STORAGE_KEY]).toBeUndefined();
+    expect(mockUpdate).toHaveBeenCalledWith({ language: null });
+    expect(i18nMock.changeLanguage).toHaveBeenCalledWith("es"); // re-detected
+    expect(result.current.hasExplicitChoice).toBe(false);
+  });
+
+  it("setLanguage marks the choice explicit; clearLanguage unmarks it", () => {
+    const { result } = renderHook(() => useLanguage());
+    expect(result.current.hasExplicitChoice).toBe(false);
+    act(() => result.current.setLanguage("fr"));
+    expect(result.current.hasExplicitChoice).toBe(true);
+    act(() => result.current.clearLanguage());
+    expect(result.current.hasExplicitChoice).toBe(false);
+  });
+
   it("degrades gracefully when localStorage is unavailable", () => {
     vi.stubGlobal("localStorage", {
       getItem: () => null,
diff --git a/packages/dashboard/app/hooks/useLanguage.ts b/packages/dashboard/app/hooks/useLanguage.ts
index 37b4f37146..841f77258c 100644
--- a/packages/dashboard/app/hooks/useLanguage.ts
+++ b/packages/dashboard/app/hooks/useLanguage.ts
@@ -1,4 +1,5 @@
 import { DEFAULT_LOCALE, isLocale, type Locale, SUPPORTED_LOCALES } from "@fusion/core";
+import { normalizeToSupportedLocale } from "@fusion/i18n/config";
 import { useCallback, useEffect, useRef, useState } from "react";
 import { useTranslation } from "react-i18next";
 import { fetchGlobalSettings, updateGlobalSettings } from "../api";
@@ -16,10 +17,26 @@ function readCachedLanguage(): Locale | undefined {
   }
 }
 
+/** Re-run browser language detection, mirroring the i18next detector's
+ *  navigator step (used when the user clears their explicit choice). */
+function detectBrowserLocale(): Locale {
+  if (!isBrowser) return DEFAULT_LOCALE;
+  for (const candidate of navigator.languages ?? [navigator.language]) {
+    const match = normalizeToSupportedLocale(candidate);
+    if (match) return match;
+  }
+  return DEFAULT_LOCALE;
+}
+
 export interface UseLanguageReturn {
   language: Locale;
   supportedLocales: readonly Locale[];
   setLanguage: (locale: Locale) => void;
+  /** Drop the explicit choice everywhere (localStorage + server) and revert
+   *  to runtime auto-detection. */
+  clearLanguage: () => void;
+  /** True when the user has explicitly chosen a language (vs auto-detect). */
+  hasExplicitChoice: boolean;
 }
 
 /**
@@ -36,6 +53,9 @@ export function useLanguage(): UseLanguageReturn {
   const resolved = i18n.resolvedLanguage ?? i18n.language;
   const initial: Locale = isLocale(resolved) ? resolved : DEFAULT_LOCALE;
   const [language, setLanguageState] = useState<Locale>(initial);
+  const [hasExplicitChoice, setHasExplicitChoice] = useState<boolean>(
+    () => readCachedLanguage() !== undefined,
+  );
   const userSetRef = useRef(false);
 
   // Reflect any i18next language change (including programmatic ones) into state.
@@ -54,12 +74,19 @@ export function useLanguage(): UseLanguageReturn {
   useEffect(() => {
     if (!isBrowser) return;
     const onStorage = (event: StorageEvent) => {
-      if (
-        event.key === LANGUAGE_STORAGE_KEY &&
-        isLocale(event.newValue) &&
-        event.newValue !== i18n.resolvedLanguage
-      ) {
-        void i18n.changeLanguage(event.newValue);
+      if (event.key !== LANGUAGE_STORAGE_KEY) return;
+      if (isLocale(event.newValue)) {
+        setHasExplicitChoice(true);
+        if (event.newValue !== i18n.resolvedLanguage) {
+          void i18n.changeLanguage(event.newValue);
+        }
+      } else if (event.newValue === null) {
+        // Another tab cleared the choice — revert to auto-detect here too.
+        setHasExplicitChoice(false);
+        const detected = detectBrowserLocale();
+        if (detected !== i18n.resolvedLanguage) {
+          void i18n.changeLanguage(detected);
+        }
       }
     };
     window.addEventListener("storage", onStorage);
@@ -94,8 +121,9 @@ export function useLanguage(): UseLanguageReturn {
     (locale: Locale) => {
       userSetRef.current = true;
       setLanguageState(locale);
-      // changeLanguage re-renders the tree in place and the detector caches the
-      // choice to localStorage; write it explicitly too in case caching is off.
+      setHasExplicitChoice(true);
+      // changeLanguage re-renders the tree in place; the storage key is the
+      // marker for "explicit user choice", written only here.
       void i18n.changeLanguage(locale);
       try {
         localStorage.setItem(LANGUAGE_STORAGE_KEY, locale);
@@ -109,5 +137,23 @@ export function useLanguage(): UseLanguageReturn {
     [i18n],
   );
 
-  return { language, supportedLocales: SUPPORTED_LOCALES, setLanguage };
+  const clearLanguage = useCallback(() => {
+    userSetRef.current = true;
+    setHasExplicitChoice(false);
+    try {
+      localStorage.removeItem(LANGUAGE_STORAGE_KEY);
+    } catch {
+      // localStorage unavailable — server clear below still runs.
+    }
+    // `language: null` is null-as-delete at the store boundary: the persisted
+    // key is removed and every surface reverts to runtime auto-detection.
+    void updateGlobalSettings({ language: null } as unknown as Parameters<
+      typeof updateGlobalSettings
+    >[0]).catch((error) => {
+      console.warn("[useLanguage] Failed to clear language in global settings", error);
+    });
+    void i18n.changeLanguage(detectBrowserLocale());
+  }, [i18n]);
+
+  return { language, supportedLocales: SUPPORTED_LOCALES, setLanguage, clearLanguage, hasExplicitChoice };
 }
diff --git a/packages/dashboard/app/i18n/format.ts b/packages/dashboard/app/i18n/format.ts
index 75231cb755..ddcecc417f 100644
--- a/packages/dashboard/app/i18n/format.ts
+++ b/packages/dashboard/app/i18n/format.ts
@@ -1,3 +1,4 @@
+import { useMemo } from "react";
 import { useTranslation } from "react-i18next";
 
 /**
@@ -11,15 +12,20 @@ export function useLocaleFormat() {
   const { i18n } = useTranslation();
   const locale = i18n.resolvedLanguage || i18n.language || "en";
 
-  return {
-    locale,
-    formatDate: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
-      new Date(value).toLocaleDateString(locale, options),
-    formatTime: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
-      new Date(value).toLocaleTimeString(locale, options),
-    formatDateTime: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
-      new Date(value).toLocaleString(locale, options),
-    formatNumber: (value: number, options?: Intl.NumberFormatOptions) =>
-      value.toLocaleString(locale, options),
-  };
+  // Memoized per locale so the formatter identities stay stable across
+  // renders — consumers can safely put them in hook dependency arrays.
+  return useMemo(
+    () => ({
+      locale,
+      formatDate: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
+        new Date(value).toLocaleDateString(locale, options),
+      formatTime: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
+        new Date(value).toLocaleTimeString(locale, options),
+      formatDateTime: (value: number | string | Date, options?: Intl.DateTimeFormatOptions) =>
+        new Date(value).toLocaleString(locale, options),
+      formatNumber: (value: number, options?: Intl.NumberFormatOptions) =>
+        value.toLocaleString(locale, options),
+    }),
+    [locale],
+  );
 }
diff --git a/packages/i18n/locales/en/app.json b/packages/i18n/locales/en/app.json
index 63cf775b30..bd4d576055 100644
--- a/packages/i18n/locales/en/app.json
+++ b/packages/i18n/locales/en/app.json
@@ -3,6 +3,8 @@
     "appearance": {
       "title": "Appearance",
       "language": "Language",
+      "languageAuto": "Auto",
+      "languageAutoHint": "Follow the browser language",
       "languageHint": "Choose the language for the {{brand}} interface."
     }
   }
diff --git a/packages/i18n/locales/es/app.json b/packages/i18n/locales/es/app.json
index 02f8b142a8..04d4be5c92 100644
--- a/packages/i18n/locales/es/app.json
+++ b/packages/i18n/locales/es/app.json
@@ -3,6 +3,8 @@
     "appearance": {
       "title": "Apariencia",
       "language": "Idioma",
+      "languageAuto": "Automático",
+      "languageAutoHint": "Seguir el idioma del navegador",
       "languageHint": "Elige el idioma de la interfaz de {{brand}}."
     }
   }
diff --git a/packages/i18n/locales/fr/app.json b/packages/i18n/locales/fr/app.json
index daae4e691f..7345b8f42c 100644
--- a/packages/i18n/locales/fr/app.json
+++ b/packages/i18n/locales/fr/app.json
@@ -3,6 +3,8 @@
     "appearance": {
       "title": "Apparence",
       "language": "Langue",
+      "languageAuto": "Auto",
+      "languageAutoHint": "Suivre la langue du navigateur",
       "languageHint": "Choisissez la langue de l'interface {{brand}}."
     }
   }
diff --git a/packages/i18n/locales/zh-CN/app.json b/packages/i18n/locales/zh-CN/app.json
index 83ce6b3ad0..258909bb4c 100644
--- a/packages/i18n/locales/zh-CN/app.json
+++ b/packages/i18n/locales/zh-CN/app.json
@@ -3,6 +3,8 @@
     "appearance": {
       "title": "外观",
       "language": "语言",
+      "languageAuto": "自动",
+      "languageAutoHint": "跟随浏览器语言",
       "languageHint": "选择 {{brand}} 界面的语言。"
     }
   }
diff --git a/packages/i18n/locales/zh-TW/app.json b/packages/i18n/locales/zh-TW/app.json
index 1533de30b4..e5f40dbbf6 100644
--- a/packages/i18n/locales/zh-TW/app.json
+++ b/packages/i18n/locales/zh-TW/app.json
@@ -3,6 +3,8 @@
     "appearance": {
       "title": "外觀",
       "language": "語言",
+      "languageAuto": "自動",
+      "languageAutoHint": "跟隨瀏覽器語言",
       "languageHint": "選擇 {{brand}} 介面的語言。"
     }
   }