1 files changed, 137 insertions, 0 deletions
diff --git a/Jellyfin.Server/Middleware/AcceptLanguageMiddleware.cs b/Jellyfin.Server/Middleware/AcceptLanguageMiddleware.cs
new file mode 100644
index 0000000000..57390ae005
--- /dev/null
+++ b/Jellyfin.Server/Middleware/AcceptLanguageMiddleware.cs
@@ -0,0 +1,137 @@
+using System;
+using System.Collections.Generic;
+using System.Globalization;
+using System.Linq;
+using System.Threading.Tasks;
+using Emby.Server.Implementations.Localization;
+using MediaBrowser.Controller.Configuration;
+using Microsoft.AspNetCore.Http;
+
+namespace Jellyfin.Server.Middleware;
+
+/// <summary>
+/// Middleware that resolves the <c>Accept-Language</c> request header
+/// to an ordered list of Jellyfin-supported cultures, sets the fallback chain
+/// on <see cref="LocalizationManager.RequestCultureFallback"/>, and writes
+/// the <c>Content-Language</c> response header.
+/// </summary>
+public class AcceptLanguageMiddleware
+{
+    private readonly RequestDelegate _next;
+
+    /// <summary>
+    /// Initializes a new instance of the <see cref="AcceptLanguageMiddleware"/> class.
+    /// </summary>
+    /// <param name="next">Next request delegate.</param>
+    public AcceptLanguageMiddleware(RequestDelegate next)
+    {
+        _next = next;
+    }
+
+    /// <summary>
+    /// Invoke request.
+    /// </summary>
+    /// <param name="context">Request context.</param>
+    /// <param name="configurationManager">The server configuration manager.</param>
+    /// <returns>Task.</returns>
+    public async Task Invoke(HttpContext context, IServerConfigurationManager configurationManager)
+    {
+        var chain = ResolveLanguages(context.Request, configurationManager);
+        if (chain is not null)
+        {
+            LocalizationManager.RequestCultureFallback = chain;
+            CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo(chain[0]);
+        }
+
+        context.Response.OnStarting(
+            static state =>
+            {
+                var (ctx, languages) = ((HttpContext, IReadOnlyList<string>?))state;
+                if (languages is not null)
+                {
+                    ctx.Response.Headers.ContentLanguage = string.Join(", ", languages);
+                }
+                else
+                {
+                    var culture = CultureInfo.CurrentUICulture.Name;
+                    if (!string.IsNullOrEmpty(culture))
+                    {
+                        ctx.Response.Headers.ContentLanguage = culture;
+                    }
+                }
+
+                return Task.CompletedTask;
+            },
+            (context, chain));
+
+        try
+        {
+            await _next(context).ConfigureAwait(false);
+        }
+        finally
+        {
+            LocalizationManager.RequestCultureFallback = null;
+        }
+    }
+
+    private static IReadOnlyList<string>? ResolveLanguages(HttpRequest request, IServerConfigurationManager configurationManager)
+    {
+        var acceptLanguageHeader = request.GetTypedHeaders().AcceptLanguage;
+        if (acceptLanguageHeader is null || acceptLanguageHeader.Count == 0)
+        {
+            return null;
+        }
+
+        var languages = acceptLanguageHeader
+            .OrderByDescending(h => h.Quality ?? 1)
+            .Select(h => h.Value.ToString());
+
+        var chain = new List<string>();
+        var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
+        foreach (var lang in languages)
+        {
+            TryAddCulture(lang, chain, seen);
+        }
+
+        // Append server default culture if not already present
+        var serverCulture = configurationManager.Configuration.UICulture;
+        if (!string.IsNullOrEmpty(serverCulture))
+        {
+            TryAddCulture(serverCulture, chain, seen);
+        }
+
+        // Ensure en-US is always the final fallback
+        TryAddCulture("en-US", chain, seen);
+
+        return chain;
+    }
+
+    private static void TryAddCulture(string lang, List<string> chain, HashSet<string> seen)
+    {
+        // Direct match
+        if (LocalizationManager.HasTranslation(lang) && seen.Add(lang))
+        {
+            chain.Add(lang);
+            return;
+        }
+
+        // BCP-47 to Jellyfin underscore mapping (e.g. es-419 -> es_419)
+        if (LocalizationManager.Bcp47ToJellyfinMap.TryGetValue(lang, out var mapped) && seen.Add(mapped))
+        {
+            chain.Add(mapped);
+            return;
+        }
+
+        // Parent culture fallback (e.g. de-DE -> de)
+        var dashIndex = lang.IndexOf('-', StringComparison.Ordinal);
+        if (dashIndex > 0)
+        {
+            var parent = lang[..dashIndex];
+            if (LocalizationManager.HasTranslation(parent) && seen.Add(parent))
+            {
+                chain.Add(parent);
+            }
+        }
+    }
+}