fix: default setup language to system locale (#500)

tests/unit/language-detection.test.ts

@@ -0,0 +1,20 @@
+import { describe, expect, it } from 'vitest';
+import { resolveSupportedLanguage } from '../../shared/language';
+
+describe('resolveSupportedLanguage', () => {
+  it('uses the base language for supported regional locales', () => {
+    expect(resolveSupportedLanguage('zh-CN')).toBe('zh');
+    expect(resolveSupportedLanguage('ja_JP')).toBe('ja');
+    expect(resolveSupportedLanguage('en-US')).toBe('en');
+  });
+
+  it('falls back to English for unsupported locales', () => {
+    expect(resolveSupportedLanguage('fr-FR')).toBe('en');
+    expect(resolveSupportedLanguage('ko')).toBe('en');
+  });
+
+  it('falls back to English when locale is missing', () => {
+    expect(resolveSupportedLanguage('')).toBe('en');
+    expect(resolveSupportedLanguage(undefined)).toBe('en');
+  });
+});