feat: mail service

authsession/internal/service/shared/preferred_language.go

@@ -0,0 +1,27 @@
+package shared
+
+import "golang.org/x/text/language"
+
+const defaultPreferredLanguage = "en"
+
+// ResolvePreferredLanguage returns the first canonical BCP 47 language tag
+// accepted from value, or the stable "en" fallback when the input is absent,
+// malformed, or too unspecific for auth registration purposes.
+func ResolvePreferredLanguage(value string) string {
+	tags, _, err := language.ParseAcceptLanguage(value)
+	if err != nil {
+		return defaultPreferredLanguage
+	}
+
+	for _, tag := range tags {
+		canonical := tag.String()
+		switch canonical {
+		case "", "und", "mul":
+			continue
+		default:
+			return canonical
+		}
+	}
+
+	return defaultPreferredLanguage
+}