merged from gramsrv upstream

internal/domain/client_session.go

@@ -0,0 +1,28 @@
+package domain
+
+import "strings"
+
+// ClientSessionMetadata is request-scoped initConnection/session context for
+// in-process features such as localized service bots. It is deliberately not a
+// durable message field and must never be exposed through the public Bot API.
+type ClientSessionMetadata struct {
+	AuthKeyID      [8]byte
+	SessionID      int64
+	SystemLangCode string
+	LangPack       string
+	LangCode       string
+}
+
+// PreferredLanguage returns a normalized BCP-47 primary language subtag.
+// Telegram clients normally send lang_code, with system_lang_code as fallback.
+func (m ClientSessionMetadata) PreferredLanguage() string {
+	value := strings.TrimSpace(m.LangCode)
+	if value == "" {
+		value = strings.TrimSpace(m.SystemLangCode)
+	}
+	value = strings.ToLower(strings.ReplaceAll(value, "_", "-"))
+	if at := strings.IndexByte(value, '-'); at >= 0 {
+		value = value[:at]
+	}
+	return value
+}