Add localization support

Co-authored-by: Xiang <xfqz86@163.com>

Library/Database/SharedPreferences.swift

@@ -1,6 +1,8 @@
 import Foundation
 
 public enum SharedPreferences {
+    public static let language = Preference<String>("language", defaultValue: "")
+
     public static let selectedProfileID = Preference<Int64>("selected_profile_id", defaultValue: -1)
 
     #if os(macOS)

Library/Network/HTTPClient.swift

@@ -10,6 +10,8 @@ public class HTTPClient {
         userAgent += Bundle.main.versionNumber
         userAgent += "; sing-box "
         userAgent += LibboxVersion()
+        userAgent += "; language "
+        userAgent += Locale.current.identifier
         userAgent += ")"
         return userAgent
     }