UserLanguage not null

src/Infrastructure/BotSharp.Abstraction/Users/IUserIdentity.cs

@@ -8,7 +8,10 @@ public interface IUserIdentity
     string FirstName { get; }
     string LastName { get; }
     string FullName { get; }
-    string? UserLanguage { get; }
+    /// <summary>
+    ///  "en-US", "zh-CN"
+    /// </summary>
+    string UserLanguage { get; }
     string? Phone { get; }
     string? AffiliateId { get; }
 }

src/Infrastructure/BotSharp.Core/Users/Services/UserIdentity.cs

@@ -58,12 +58,12 @@ public class UserIdentity : IUserIdentity
     }
 
     [JsonPropertyName("user_language")]
-    public string? UserLanguage
+    public string UserLanguage
     {
         get
         {
             _contextAccessor.HttpContext.Request.Headers.TryGetValue("User-Language", out var languages);
-            return languages.FirstOrDefault();
+            return languages.FirstOrDefault() ?? "en-US"; 
         }
     }