Country Codes for Geolocation and Locale Detection

Multi-Signal Country Detection

Reliable country detection combines multiple signals rather than relying on a single source. Each signal has different accuracy and availability characteristics.

Detection Signals

Signal	Accuracy	Availability	Example
IP geolocation	95-99%	Server-side	CF-IPCountry: JP
Browser locale	High (user-set)	Client-side	navigator.language = "ja-JP"
Timezone	~90%	Client-side	Asia/Tokyo → JP
Accept-Language	High (user-set)	Both	Accept-Language: ja-JP,ja;q=0.9
GPS (mobile)	Very high	Permission needed	Lat/Long → Reverse geocode

Implementation Pattern

function detectCountry(req) {
  // 1. Explicit user preference (highest priority)
  const saved = getCookie('preferred_country');
  if (saved) return saved;

  // 2. CDN geolocation header
  const cfCountry = req.headers['cf-ipcountry'];
  if (cfCountry && cfCountry !== 'XX') return cfCountry;

  // 3. Accept-Language header
  const lang = req.headers['accept-language'];
  const match = lang?.match(/[a-z]{2}-([A-Z]{2})/);
  if (match) return match[1];

  // 4. Default
  return 'US';
}

Timezone to Country Mapping

Many IANA timezones map to a single country:

const TZ_TO_COUNTRY = {
  'Asia/Tokyo': 'JP',
  'Asia/Seoul': 'KR',
  'Europe/London': 'GB',
  'Europe/Berlin': 'DE',
  'Europe/Paris': 'FR',
  'America/New_York': 'US',
  'America/Toronto': 'CA',
  'Australia/Sydney': 'AU',
  'Pacific/Auckland': 'NZ',
};

function countryFromTimezone() {
  const tz = Intl.DateTimeFormat().resolvedOptions().timeZone;
  return TZ_TO_COUNTRY[tz] || null;
}

However, some timezones are shared:

• America/Chicago is used in both the US and... only the US, but with multiple states
• Europe/Berlin covers Germany, but also historical default for several Central European countries
• Asia/Kolkata = India (IN), Asia/Colombo = Sri Lanka (LK) — these are different

Client-Side Detection

function detectCountryClient() {
  // Language-based
  const locale = navigator.language || navigator.userLanguage;
  const parts = locale.split('-');
  if (parts.length >= 2) {
    return parts[1].toUpperCase(); // "en-US" → "US"
  }

  // Timezone-based fallback
  return countryFromTimezone();
}

Best Practices

1. Always allow manual override — Users may be traveling or using VPNs
2. Store the preference — Save the selected country in a cookie or account settings
3. Combine signals — Use IP + locale + timezone for higher confidence
4. Respect privacy — Do not require GPS for basic localization
5. Handle edge cases — VPN users, proxies, shared IPs (cloud, corporate)