Add automatic language detection from Accept-Language header

[Copilot]

The documentation site hardcoded a redirect to /en/docs, ignoring users' browser language preferences.

Implementation

Modified root page to parse Accept-Language header and redirect to best matching locale:

// Before: hardcoded English
redirect('/en/docs');

// After: respects browser preferences
const headersList = await headers();
const acceptLanguage = headersList.get('accept-language') || '';
const preferredLang = getPreferredLanguage(acceptLanguage, i18n.languages);
redirect(`/${preferredLang}/docs`);

Language Matching Logic

• Parses quality values (zh-CN;q=0.9,en;q=0.8) and sorts by preference
• Exact match first (e.g., zh-CN)
• Falls back to prefix match (e.g., zh → zh-CN)
• Validates quality parameters and filters invalid entries
• Defaults to English when no match found

Behavior

• Accept-Language: zh-CN,zh;q=0.9,en;q=0.8 → /zh-CN/docs
• Accept-Language: en-US,en;q=0.9 → /en/docs
• Accept-Language: fr-FR → /en/docs (fallback)
• No header → /en/docs (fallback)

Original prompt

文档站点默认为英文，应该能默自动识别当前用户的语言并跳转

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Review	Updated (UTC)
docs	Ready	Preview, Comment	Jan 16, 2026 10:17am

[github-actions]

🚀 Preview Deployment

This pull request will be automatically deployed to Vercel.

Preview Links

• 📝 Documentation: Will be available once Vercel deployment completes
• 🌍 Languages: English (/en/docs) and Chinese (/zh-CN/docs)

Build Status

Check the CI workflow for build status and any errors.

---

Automated preview information for PR #29

[Copilot]

Pull request overview

This PR adds automatic language detection to redirect users to documentation in their preferred language based on the Accept-Language HTTP header, replacing the previous hardcoded English redirect.

Changes:

• Modified root page handler to be async and parse the Accept-Language header
• Implemented getPreferredLanguage function with quality value parsing and locale matching logic
• Added fallback behavior to default language when no match is found

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

Case-sensitive comparison may fail to match language codes. The Accept-Language header can contain mixed case (e.g., 'en-US', 'EN-us'), but this performs exact case-sensitive matching. Consider normalizing both the lang.code and availableLanguages entries to lowercase before comparison to ensure reliable matching.

Suggested change

	// Try exact match first
	if (availableLanguages.includes(lang.code)) {
	return lang.code;
	// Try exact match first (case-insensitive)
	const exactMatch = availableLanguages.find(
	available => available.toLowerCase() === lang.code.toLowerCase()
	);
	if (exactMatch) {
	return exactMatch;

[Copilot]

The getPreferredLanguage function lacks test coverage for critical edge cases including empty Accept-Language headers, malformed quality values, case sensitivity handling, and fallback behavior. Consider adding unit tests to verify correct behavior across these scenarios.