Add Accept-Language header support and cleanup translations (#16488)

* Add Accept-Language header support for per-request localization

* Use native middleware

* Cleanup

* Add Fallback

* Build BCP47 map reflexively

* Address review comments

2 files changed, 24 insertions, 2 deletions

diff --git a/Emby.Server.Implementations/HttpServer/WebSocketConnection.cs b/Emby.Server.Implementations/HttpServer/WebSocketConnection.cs
index 373b0994a6..e9bf3b93a7 100644
--- a/Emby.Server.Implementations/HttpServer/WebSocketConnection.cs
+++ b/Emby.Server.Implementations/HttpServer/WebSocketConnection.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Buffers;
+using System.Globalization;
 using System.IO.Pipelines;
 using System.Net;
 using System.Net.WebSockets;
@@ -69,6 +70,11 @@ namespace Emby.Server.Implementations.HttpServer
         /// <inheritdoc />
         public IPAddress? RemoteEndPoint { get; }
 
+        /// <summary>
+        /// Gets or initializes the UI culture captured from the upgrade request.
+        /// </summary>
+        public CultureInfo? RequestUICulture { get; init; }
+
         /// <inheritdoc />
         public Func<WebSocketMessageInfo, Task>? OnReceive { get; set; }
 
@@ -82,6 +88,17 @@ namespace Emby.Server.Implementations.HttpServer
         public WebSocketState State => _socket.State;
 
         /// <inheritdoc />
+        public void ApplyRequestCulture()
+        {
+            if (RequestUICulture is null)
+            {
+                return;
+            }
+
+            CultureInfo.CurrentUICulture = RequestUICulture;
+        }
+
+        /// <inheritdoc />
         public async Task SendAsync(OutboundWebSocketMessage message, CancellationToken cancellationToken)
         {
             var json = JsonSerializer.SerializeToUtf8Bytes(message, _jsonOptions);
diff --git a/Emby.Server.Implementations/HttpServer/WebSocketManager.cs b/Emby.Server.Implementations/HttpServer/WebSocketManager.cs
index cb5b3993b8..072034c4bf 100644
--- a/Emby.Server.Implementations/HttpServer/WebSocketManager.cs
+++ b/Emby.Server.Implementations/HttpServer/WebSocketManager.cs
@@ -4,6 +4,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.Globalization;
 using System.Linq;
 using System.Net.WebSockets;
 using System.Threading.Tasks;
@@ -47,14 +48,18 @@ namespace Emby.Server.Implementations.HttpServer
                 _logger.LogInformation("WS {IP} request", context.Connection.RemoteIpAddress);
 
                 WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync().ConfigureAwait(false);
-
                 var connection = new WebSocketConnection(
                     _loggerFactory.CreateLogger<WebSocketConnection>(),
                     webSocket,
                     authorizationInfo,
                     context.GetNormalizedRemoteIP())
                 {
-                    OnReceive = ProcessWebSocketMessageReceived
+                    RequestUICulture = CultureInfo.CurrentUICulture
+                };
+                connection.OnReceive = result =>
+                {
+                    connection.ApplyRequestCulture();
+                    return ProcessWebSocketMessageReceived(result);
                 };
                 await using (connection.ConfigureAwait(false))
                 {