feat: backend service

backend/internal/geo/country_languages.go

@@ -0,0 +1,63 @@
+package geo
+
+import "strings"
+
+// countryToLanguage maps an uppercase ISO 3166-1 alpha-2 country code to
+// an ISO 639-1 lowercase language code. The set is intentionally minimal
+// — covering the top-traffic Galaxy locales — and is consulted as a
+// fallback when neither the request body nor the Accept-Language header
+// supplied a locale at send-email-code. Unknown countries map to the
+// empty string so the auth flow can default to "en".
+//
+// The mapping is intentionally hard-coded rather than derived from the
+// GeoLite2 database: countries with multiple official languages collapse
+// to the single most common UI locale to keep the registration path
+// deterministic. The implementation may revise this table without changing the
+// surface auth depends on.
+var countryToLanguage = map[string]string{
+	// English-default territories and the platform fallback.
+	"US": "en", "GB": "en", "AU": "en", "NZ": "en", "IE": "en", "CA": "en",
+	// Western Europe.
+	"DE": "de", "AT": "de", "CH": "de",
+	"FR": "fr", "BE": "fr", "LU": "fr",
+	"ES": "es", "MX": "es", "AR": "es", "CL": "es", "CO": "es",
+	"IT": "it",
+	"PT": "pt", "BR": "pt",
+	"NL": "nl",
+	// Central / Eastern Europe.
+	"PL": "pl",
+	"RU": "ru", "BY": "ru", "KZ": "ru",
+	"UA": "uk",
+	"CZ": "cs",
+	"SK": "sk",
+	"HU": "hu",
+	"RO": "ro",
+	"BG": "bg",
+	// Northern Europe.
+	"SE": "sv",
+	"NO": "no",
+	"DK": "da",
+	"FI": "fi",
+	// Asia.
+	"JP": "ja",
+	"KR": "ko",
+	"CN": "zh", "TW": "zh", "HK": "zh", "SG": "zh",
+	"VN": "vi",
+	"TH": "th",
+	"ID": "id",
+	"IN": "en",
+	"IL": "he",
+	"TR": "tr",
+	// Middle East and North Africa.
+	"SA": "ar", "AE": "ar", "EG": "ar",
+}
+
+// languageForCountry returns the ISO 639-1 language code mapped to
+// country, or "" when no mapping is known. country is normalised to
+// uppercase before lookup.
+func languageForCountry(country string) string {
+	if country == "" {
+		return ""
+	}
+	return countryToLanguage[strings.ToUpper(strings.TrimSpace(country))]
+}