feat: i18n support with first translations

tests/renderer/i18n.test.ts

@@ -0,0 +1,27 @@
+import { describe, expect, it } from 'vitest';
+import {
+  translateUi,
+  resolveSupportedUiLanguage,
+  resolveUiLanguageFromSystemLocale,
+} from '../../src/renderer/i18n';
+
+describe('renderer i18n', () => {
+  it('resolves supported ui language from OS locale', () => {
+    expect(resolveUiLanguageFromSystemLocale('de-DE')).toBe('de');
+    expect(resolveUiLanguageFromSystemLocale('fr-CH')).toBe('fr');
+    expect(resolveUiLanguageFromSystemLocale('pt-BR')).toBe('en');
+  });
+
+  it('normalizes explicit ui language values', () => {
+    expect(resolveSupportedUiLanguage('it')).toBe('it');
+    expect(resolveSupportedUiLanguage('es-MX')).toBe('es');
+    expect(resolveSupportedUiLanguage('')).toBe('en');
+  });
+
+  it('returns translated text with english fallback', () => {
+    expect(translateUi('de', 'common.save')).toBe('Speichern');
+    expect(translateUi('fr', 'common.cancel')).toBe('Annuler');
+    expect(translateUi('de', 'settings.language.english')).toBe('Englisch');
+    expect(translateUi('it', 'missing.key')).toBe('missing.key');
+  });
+});