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

1 files changed, 93 insertions, 0 deletions

diff --git a/tests/Jellyfin.Server.Implementations.Tests/Localization/LocalizationManagerTests.cs b/tests/Jellyfin.Server.Implementations.Tests/Localization/LocalizationManagerTests.cs
index acabaf3acb..3b8fe5ca60 100644
--- a/tests/Jellyfin.Server.Implementations.Tests/Localization/LocalizationManagerTests.cs
+++ b/tests/Jellyfin.Server.Implementations.Tests/Localization/LocalizationManagerTests.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Globalization;
 using System.Linq;
 using System.Threading.Tasks;
 using BitFaster.Caching;
@@ -305,6 +306,98 @@ namespace Jellyfin.Server.Implementations.Tests.Localization
             Assert.Equal(key, translated);
         }
 
+        [Fact]
+        public void GetLocalizedString_WithCulture_ReturnsTranslation()
+        {
+            var localizationManager = Setup(new ServerConfiguration
+            {
+                UICulture = "en-US"
+            });
+
+            var translated = localizationManager.GetLocalizedString("Artists", "de");
+            Assert.Equal("Interpreten", translated);
+        }
+
+        [Fact]
+        public void GetLocalizedString_WithCulture_FallsBackToEnUs()
+        {
+            var localizationManager = Setup(new ServerConfiguration
+            {
+                UICulture = "en-US"
+            });
+
+            // A culture with no translation file should fall back to en-US
+            var translated = localizationManager.GetLocalizedString("Artists", "zz");
+            Assert.Equal("Artists", translated);
+        }
+
+        [Fact]
+        public void GetLocalizedString_WithBcp47Normalization_ReturnsTranslation()
+        {
+            var localizationManager = Setup(new ServerConfiguration
+            {
+                UICulture = "en-US"
+            });
+
+            // es-419 is stored as es_419 in Jellyfin
+            var translated = localizationManager.GetLocalizedString("Default", "es-419");
+            Assert.NotEqual("Default", translated);
+        }
+
+        [Fact]
+        public void GetServerLocalizedString_UsesServerCulture()
+        {
+            var localizationManager = Setup(new ServerConfiguration
+            {
+                UICulture = "de"
+            });
+
+            // Even if CurrentUICulture is fr, GetServerLocalizedString should use the server's "de"
+            var previousCulture = CultureInfo.CurrentUICulture;
+            try
+            {
+                CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo("fr");
+                var translated = localizationManager.GetServerLocalizedString("Artists");
+                Assert.Equal("Interpreten", translated);
+            }
+            finally
+            {
+                CultureInfo.CurrentUICulture = previousCulture;
+            }
+        }
+
+        [Fact]
+        public void GetLocalizedString_UsesCurrentUICulture()
+        {
+            var localizationManager = Setup(new ServerConfiguration
+            {
+                UICulture = "en-US"
+            });
+
+            var previousCulture = CultureInfo.CurrentUICulture;
+            try
+            {
+                CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo("de");
+                var translated = localizationManager.GetLocalizedString("Artists");
+                Assert.Equal("Interpreten", translated);
+            }
+            finally
+            {
+                CultureInfo.CurrentUICulture = previousCulture;
+            }
+        }
+
+        [Fact]
+        public void GetSupportedUICultures_IncludesCommonCultures()
+        {
+            var supported = LocalizationManager.GetSupportedUICultures();
+            Assert.Contains(supported, c => c.Name.Equals("de", StringComparison.OrdinalIgnoreCase));
+            Assert.Contains(supported, c => c.Name.Equals("en-US", StringComparison.OrdinalIgnoreCase));
+            Assert.Contains(supported, c => c.Name.Equals("fr", StringComparison.OrdinalIgnoreCase));
+            // Underscore variants get normalized to BCP-47 hyphen form for CultureInfo compatibility.
+            Assert.Contains(supported, c => c.Name.Equals("es-419", StringComparison.OrdinalIgnoreCase));
+        }
+
         private LocalizationManager Setup(ServerConfiguration config)
         {
             var mockConfiguration = new Mock<IServerConfigurationManager>();