Add Accept-Language header support for per-request localization

MediaBrowser.Controller/Net/BasePeriodicWebSocketListener.cs

@@ -209,6 +209,11 @@ namespace MediaBrowser.Controller.Net
                 var (connection, cts, state) = tuple;
                 var cancellationToken = cts.Token;
 
+                // Restore the culture context captured when the connection was established
+                // so that GetDataToSendForConnection produces a localized payload matching
+                // the client's Accept-Language preference rather than the server default.
+                connection.ApplyRequestCulture();
+
                 var data = await GetDataToSendForConnection(connection).ConfigureAwait(false);
                 if (data is null)
                 {

MediaBrowser.Controller/Net/IWebSocketConnection.cs

@@ -77,5 +77,14 @@ namespace MediaBrowser.Controller.Net
         /// <param name="cancellationToken">The cancellation token.</param>
         /// <returns>Task.</returns>
         Task ReceiveAsync(CancellationToken cancellationToken = default);
+
+        /// <summary>
+        /// Applies the culture context captured when the connection was established
+        /// (from the upgrade request's <c>Accept-Language</c> header) to the current
+        /// async flow. Server-initiated message senders should call this before
+        /// localising any payload so that the response uses the client's preferred
+        /// language rather than the server default.
+        /// </summary>
+        void ApplyRequestCulture();
     }
 }