diff --git a/src/localize/localize.ts b/src/localize/localize.ts
index 113fef10..2c9d251a 100644
--- a/src/localize/localize.ts
+++ b/src/localize/localize.ts
@@ -70,9 +70,7 @@ export const loadLanguages = async (hass: HomeAssistant): Promise<boolean> => {
     languages[lang] = await import('./languages/pt-BR.json');
   }
 
-  if (lang) {
-    advancedCameraCardLanguage = lang;
-  }
+  advancedCameraCardLanguage = lang;
   return true;
 };
 
diff --git a/tests/localize/localize.test.ts b/tests/localize/localize.test.ts
new file mode 100644
index 00000000..12e72b5c
--- /dev/null
+++ b/tests/localize/localize.test.ts
@@ -0,0 +1,261 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+import { type MockProxy, mock } from 'vitest-mock-extended';
+import type { HomeAssistant } from '../../src/ha/types';
+
+// localize.ts stores the active language and loaded translations in module-level
+// variables. resetModules() clears Vitest's module cache so each test gets a
+// fresh import with those variables re-initialized.
+const importFresh = async () => {
+  vi.resetModules();
+  return await import('../../src/localize/localize');
+};
+
+const stubGlobals = (): MockProxy<Storage> => {
+  const storage = mock<Storage>();
+  storage.getItem.mockReturnValue(null);
+  vi.stubGlobal('localStorage', storage);
+  vi.stubGlobal('navigator', { languages: [] });
+  return storage;
+};
+
+describe('getLanguage', () => {
+  let storage: MockProxy<Storage>;
+
+  beforeEach(() => {
+    storage = stubGlobals();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('should return hass.language when available', async () => {
+    const { getLanguage } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'fr';
+    hass.selectedLanguage = null;
+
+    expect(getLanguage(hass)).toBe('fr');
+  });
+
+  it('should return hass.selectedLanguage when language is not set', async () => {
+    const { getLanguage } = await importFresh();
+    const hass = mock<HomeAssistant>();
+
+    // language must be nullish (not empty string) for selectedLanguage fallback.
+    hass.language = undefined as unknown as string;
+    hass.selectedLanguage = 'de';
+
+    expect(getLanguage(hass)).toBe('de');
+  });
+
+  it('should canonicalize hyphens to underscores', async () => {
+    const { getLanguage } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'pt-BR';
+
+    expect(getLanguage(hass)).toBe('pt_BR');
+  });
+
+  it('should fall back to localStorage selectedLanguage', async () => {
+    const { getLanguage } = await importFresh();
+    storage.getItem.mockReturnValue(JSON.stringify('it'));
+
+    expect(getLanguage()).toBe('it');
+  });
+
+  it('should canonicalize localStorage language', async () => {
+    const { getLanguage } = await importFresh();
+    storage.getItem.mockReturnValue(JSON.stringify('pt-BR'));
+
+    expect(getLanguage()).toBe('pt_BR');
+  });
+
+  it('should skip null localStorage value', async () => {
+    const { getLanguage } = await importFresh();
+    storage.getItem.mockReturnValue(JSON.stringify(null));
+
+    expect(getLanguage()).toBe('en');
+  });
+
+  it('should fall back to navigator.languages when matching a loaded language', async () => {
+    vi.stubGlobal('navigator', { languages: ['en'] });
+    const { getLanguage } = await importFresh();
+
+    expect(getLanguage()).toBe('en');
+  });
+
+  it('should skip non-matching navigator languages', async () => {
+    vi.stubGlobal('navigator', { languages: ['xx', 'yy'] });
+    const { getLanguage } = await importFresh();
+
+    expect(getLanguage()).toBe('en');
+  });
+
+  it('should return default language when no sources available', async () => {
+    const { getLanguage } = await importFresh();
+
+    expect(getLanguage()).toBe('en');
+  });
+});
+
+describe('loadLanguages', () => {
+  beforeEach(() => {
+    stubGlobals();
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('should load and use Catalan translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'ca';
+
+    await loadLanguages(hass);
+
+    expect(localize('common.live')).toBe('En directe');
+  });
+
+  it('should load and use German translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'de';
+
+    await loadLanguages(hass);
+
+    expect(localize('actions.abort')).toBe('Aktion abbrechen');
+  });
+
+  it('should load and use French translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'fr';
+
+    await loadLanguages(hass);
+
+    expect(localize('common.live')).toBe('En direct');
+  });
+
+  it('should load and use Italian translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'it';
+
+    await loadLanguages(hass);
+
+    expect(localize('common.no_media')).toBe(
+      'Nessun contenuto multimediale da visualizzare',
+    );
+  });
+
+  it('should load and use Polish translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'pl';
+
+    await loadLanguages(hass);
+
+    expect(localize('actions.abort')).toBe('Przerwano akcję');
+  });
+
+  it('should load and use Portuguese translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'pt';
+
+    await loadLanguages(hass);
+
+    expect(localize('common.live')).toBe('Ao Vivo');
+  });
+
+  it('should load and use Brazilian Portuguese translations', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'pt_BR';
+    hass.selectedLanguage = null;
+
+    await loadLanguages(hass);
+
+    expect(localize('common.live')).toBe('Ao Vivo');
+  });
+
+  it('should fall back to English for unsupported language', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'xx';
+
+    await loadLanguages(hass);
+
+    expect(localize('common.advanced_camera_card')).toBe('Advanced Camera Card');
+  });
+});
+
+describe('localize', () => {
+  beforeEach(() => {
+    stubGlobals();
+    vi.stubGlobal('navigator', { languages: [] });
+  });
+
+  afterEach(() => {
+    vi.unstubAllGlobals();
+  });
+
+  it('should resolve a dotted key from English', async () => {
+    const { localize } = await importFresh();
+
+    expect(localize('common.advanced_camera_card')).toBe('Advanced Camera Card');
+  });
+
+  it('should use loaded language when available', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'de';
+    await loadLanguages(hass);
+
+    // German translation should be used (and it exists).
+    const result = localize('common.advanced_camera_card');
+    expect(result).toBeTruthy();
+  });
+
+  it('should fall back to English when primary language key is missing', async () => {
+    const { loadLanguages, localize } = await importFresh();
+    const hass = mock<HomeAssistant>();
+    hass.language = 'de';
+    await loadLanguages(hass);
+
+    // 'common.dismiss' exists only in English, so the German lookup throws and
+    // falls back to English.
+    expect(localize('common.dismiss')).toBe('Dismiss');
+  });
+
+  it('should perform search and replace', async () => {
+    const { localize } = await importFresh();
+
+    const result = localize('common.advanced_camera_card', 'Advanced', 'Basic');
+    expect(result).toBe('Basic Camera Card');
+  });
+
+  it('should not replace when search is empty', async () => {
+    const { localize } = await importFresh();
+
+    expect(localize('common.advanced_camera_card', '', '')).toBe('Advanced Camera Card');
+  });
+
+  it('should throw for completely missing key', async () => {
+    const { localize } = await importFresh();
+
+    // The first reduce throws (caught), the English fallback also throws
+    // (uncaught), so this will throw a TypeError.
+    expect(() => localize('nonexistent.deeply.nested.key')).toThrow();
+  });
+
+  it('should return translated value for single-segment key', async () => {
+    const { localize } = await importFresh();
+
+    // 'actions' is a top-level key that resolves to an object, not a string,
+    // but should not throw.
+    expect(localize('actions')).toBeTruthy();
+  });
+});
diff --git a/vite.config.ts b/vite.config.ts
index 66c7f436..538c20a8 100644
--- a/vite.config.ts
+++ b/vite.config.ts
@@ -9,6 +9,7 @@ const FULL_COVERAGE = [
   'src/config/**/*.ts',
   'src/const.ts',
   'src/ha/**/*.ts',
+  'src/localize/**/*.ts',
   'src/types.ts',
   'src/utils/**/*.ts',
   'src/view/*.ts',