increase test coverage

extend test coverage

tests/locale.test.js

@@ -1,7 +1,32 @@
-import { describe, expect, test } from 'vitest'
+import { afterEach, describe, expect, test } from 'vitest'
 import { putLocale, t } from '../src/editor/locale.js'
 
 const goodLangs = ['en', 'fr', 'de']
+const originalNavigator = {
+  userLanguage: navigator.userLanguage,
+  language: navigator.language
+}
+
+const setNavigatorProp = (prop, value) => {
+  Object.defineProperty(navigator, prop, {
+    value,
+    configurable: true,
+    writable: true
+  })
+}
+
+const restoreNavigatorProp = (prop, value) => {
+  if (value === undefined) {
+    Reflect.deleteProperty(navigator, prop)
+    return
+  }
+  setNavigatorProp(prop, value)
+}
+
+afterEach(() => {
+  restoreNavigatorProp('userLanguage', originalNavigator.userLanguage)
+  restoreNavigatorProp('language', originalNavigator.language)
+})
 
 describe('locale loader', () => {
   test('falls back to English when lang is not supported', async () => {
@@ -16,4 +41,22 @@ describe('locale loader', () => {
     expect(t('common.ok')).toBe('OK')
     expect(t('misc.powered_by')).toBe('Powered by')
   })
+
+  test('uses navigator.userLanguage when available', async () => {
+    setNavigatorProp('userLanguage', 'fr')
+    setNavigatorProp('language', 'en-US')
+
+    const result = await putLocale('', goodLangs)
+    expect(result.langParam).toBe('fr')
+    expect(t('common.ok')).toBe('OK')
+  })
+
+  test('uses navigator.language and still falls back to English for unsupported locale', async () => {
+    Reflect.deleteProperty(navigator, 'userLanguage')
+    setNavigatorProp('language', 'pt-BR')
+
+    const result = await putLocale('', goodLangs)
+    expect(result.langParam).toBe('en')
+    expect(t('common.ok')).toBe('OK')
+  })
 })