feat: backend service

backend/internal/geo/language.go

@@ -0,0 +1,14 @@
+package geo
+
+// LanguageForIP returns an ISO 639-1 language code derived from
+// sourceIP. The function looks up the country via LookupCountry and then
+// consults the static country->language table. Returns "" when the
+// country lookup fails or no language mapping exists for the country.
+//
+// Auth uses LanguageForIP as a fallback after the client-supplied locale
+// (request body or Accept-Language header). The empty string signals
+// "fall through to the platform default 'en'".
+func (s *Service) LanguageForIP(sourceIP string) string {
+	country := s.LookupCountry(sourceIP)
+	return languageForCountry(country)
+}