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, 144 insertions, 119 deletions

diff --git a/Emby.Server.Implementations/Localization/Core/en-US.json b/Emby.Server.Implementations/Localization/Core/en-US.json
index 9b5049c8c7..856941c61a 100644
--- a/Emby.Server.Implementations/Localization/Core/en-US.json
+++ b/Emby.Server.Implementations/Localization/Core/en-US.json
@@ -1,45 +1,29 @@
 {
-    "Albums": "Albums",
     "AppDeviceValues": "App: {0}, Device: {1}",
-    "Application": "Application",
     "Artists": "Artists",
     "AuthenticationSucceededWithUserName": "{0} successfully authenticated",
     "Books": "Books",
-    "CameraImageUploadedFrom": "A new camera image has been uploaded from {0}",
-    "Channels": "Channels",
     "ChapterNameValue": "Chapter {0}",
     "Collections": "Collections",
     "Default": "Default",
-    "DeviceOfflineWithName": "{0} has disconnected",
-    "DeviceOnlineWithName": "{0} is connected",
     "External": "External",
     "FailedLoginAttemptWithUserName": "Failed login attempt from {0}",
     "Favorites": "Favorites",
     "Folders": "Folders",
     "Forced": "Forced",
     "Genres": "Genres",
-    "HeaderAlbumArtists": "Album artists",
     "HeaderContinueWatching": "Continue Watching",
-    "HeaderFavoriteAlbums": "Favorite Albums",
-    "HeaderFavoriteArtists": "Favorite Artists",
     "HeaderFavoriteEpisodes": "Favorite Episodes",
     "HeaderFavoriteShows": "Favorite Shows",
-    "HeaderFavoriteSongs": "Favorite Songs",
     "HeaderLiveTV": "Live TV",
     "HeaderNextUp": "Next Up",
-    "HeaderRecordingGroups": "Recording Groups",
     "HearingImpaired": "Hearing Impaired",
     "HomeVideos": "Home Videos",
     "Inherit": "Inherit",
-    "ItemAddedWithName": "{0} was added to the library",
-    "ItemRemovedWithName": "{0} was removed from the library",
     "LabelIpAddressValue": "IP address: {0}",
     "LabelRunningTimeValue": "Running time: {0}",
     "Latest": "Latest",
-    "MessageApplicationUpdated": "Jellyfin Server has been updated",
-    "MessageApplicationUpdatedTo": "Jellyfin Server has been updated to {0}",
-    "MessageNamedServerConfigurationUpdatedWithValue": "Server configuration section {0} has been updated",
-    "MessageServerConfigurationUpdated": "Server configuration has been updated",
+    "LyricDownloadFailureFromForItem": "Lyrics failed to download from {0} for {1}",
     "MixedContent": "Mixed content",
     "Movies": "Movies",
     "Music": "Music",
@@ -66,24 +50,15 @@
     "NotificationOptionVideoPlaybackStopped": "Video playback stopped",
     "Original": "Original",
     "Photos": "Photos",
-    "Playlists": "Playlists",
-    "Plugin": "Plugin",
     "PluginInstalledWithName": "{0} was installed",
     "PluginUninstalledWithName": "{0} was uninstalled",
     "PluginUpdatedWithName": "{0} was updated",
-    "ProviderValue": "Provider: {0}",
     "ScheduledTaskFailedWithName": "{0} failed",
-    "ScheduledTaskStartedWithName": "{0} started",
-    "ServerNameNeedsToBeRestarted": "{0} needs to be restarted",
     "Shows": "Shows",
-    "Songs": "Songs",
     "StartupEmbyServerIsLoading": "Jellyfin Server is loading. Please try again shortly.",
     "SubtitleDownloadFailureFromForItem": "Subtitles failed to download from {0} for {1}",
-    "Sync": "Sync",
-    "System": "System",
     "TvShows": "TV Shows",
     "Undefined": "Undefined",
-    "User": "User",
     "UserCreatedWithName": "User {0} has been created",
     "UserDeletedWithName": "User {0} has been deleted",
     "UserDownloadingItemWithValues": "{0} is downloading {1}",
@@ -91,11 +66,8 @@
     "UserOfflineFromDevice": "{0} has disconnected from {1}",
     "UserOnlineFromDevice": "{0} is online from {1}",
     "UserPasswordChangedWithName": "Password has been changed for user {0}",
-    "UserPolicyUpdatedWithName": "User policy has been updated for {0}",
     "UserStartedPlayingItemWithValues": "{0} is playing {1} on {2}",
     "UserStoppedPlayingItemWithValues": "{0} has finished playing {1} on {2}",
-    "ValueHasBeenAddedToLibrary": "{0} has been added to your media library",
-    "ValueSpecialEpisodeName": "Special - {0}",
     "VersionNumber": "Version {0}",
     "TasksMaintenanceCategory": "Maintenance",
     "TasksLibraryCategory": "Library",
diff --git a/Emby.Server.Implementations/Localization/LocalizationManager.cs b/Emby.Server.Implementations/Localization/LocalizationManager.cs
index d8797e612b..0b0b300d30 100644
--- a/Emby.Server.Implementations/Localization/LocalizationManager.cs
+++ b/Emby.Server.Implementations/Localization/LocalizationManager.cs
@@ -3,6 +3,7 @@ using System.Collections.Concurrent;
 using System.Collections.Frozen;
 using System.Collections.Generic;
 using System.Diagnostics.CodeAnalysis;
+using System.Globalization;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -26,6 +27,7 @@ namespace Emby.Server.Implementations.Localization
         private const string RatingsPath = "Emby.Server.Implementations.Localization.Ratings.";
         private const string CulturesPath = "Emby.Server.Implementations.Localization.iso6392.txt";
         private const string CountriesPath = "Emby.Server.Implementations.Localization.countries.json";
+        private const string CoreResourcePrefix = "Emby.Server.Implementations.Localization.Core.";
         private static readonly Assembly _assembly = typeof(LocalizationManager).Assembly;
         private static readonly string[] _unratedValues = ["n/a", "unrated", "not rated", "nr"];
 
@@ -34,13 +36,21 @@ namespace Emby.Server.Implementations.Localization
 
         private readonly Dictionary<string, Dictionary<string, ParentalRatingScore?>> _allParentalRatings = new(StringComparer.OrdinalIgnoreCase);
 
-        private readonly ConcurrentDictionary<string, Dictionary<string, string>> _dictionaries = new(StringComparer.OrdinalIgnoreCase);
+        private readonly ConcurrentDictionary<string, Dictionary<string, string>> _cultureOnlyDictionaries = new(StringComparer.OrdinalIgnoreCase);
 
         private readonly JsonSerializerOptions _jsonOptions = JsonDefaults.Options;
 
         private readonly ConcurrentDictionary<string, CultureDto?> _cultureCache = new(StringComparer.OrdinalIgnoreCase);
         private List<CultureDto> _cultures = [];
 
+        private static readonly (IReadOnlyList<LocalizationOption> Options, FrozenDictionary<string, string> Bcp47ToJellyfinMap) _localizationData = BuildLocalizationData();
+        private static readonly IReadOnlyList<LocalizationOption> _localizationOptions = _localizationData.Options;
+
+        // Maps BCP-47 hyphenated culture codes (set by ASP.NET Core's RequestLocalizationMiddleware
+        // and used as CurrentUICulture.Name) to Jellyfin's underscore-based resource file codes.
+        // Built reflexively from the resource file scan so both directions stay in sync.
+        private static readonly FrozenDictionary<string, string> _bcp47ToJellyfinMap = _localizationData.Bcp47ToJellyfinMap;
+
         private FrozenDictionary<string, string> _iso6392BtoT = null!;
 
         /// <summary>
@@ -54,6 +64,59 @@ namespace Emby.Server.Implementations.Localization
         {
             _configurationManager = configurationManager;
             _logger = logger;
+
+            _configurationManager.ConfigurationUpdated += OnConfigurationUpdated;
+        }
+
+        /// <summary>
+        /// Gets the supported UI cultures.
+        /// </summary>
+        /// <returns>A list of <see cref="CultureInfo"/> objects covering every embedded translation.</returns>
+        public static IList<CultureInfo> GetSupportedUICultures()
+        {
+            var cultures = new List<CultureInfo>();
+            foreach (var option in _localizationOptions)
+            {
+                // Skip novelty codes (e.g. "pr" Pirate, "jbo" Lojban) that .NET cannot resolve.
+                if (TryGetCultureInfo(option.Value, out var cultureInfo))
+                {
+                    cultures.Add(cultureInfo);
+                }
+            }
+
+            return cultures;
+        }
+
+        /// <summary>
+        /// Resolves a Jellyfin resource culture code (which may use underscores, e.g. <c>es_419</c>)
+        /// to a <see cref="CultureInfo"/>. Returns <see langword="false"/> for codes .NET cannot resolve.
+        /// </summary>
+        private static bool TryGetCultureInfo(string cultureCode, [NotNullWhen(true)] out CultureInfo? cultureInfo)
+        {
+            try
+            {
+                // Resource files use underscores for some variants (e.g. es_419);
+                // CultureInfo only accepts hyphenated BCP-47 codes.
+                cultureInfo = CultureInfo.GetCultureInfo(cultureCode.Replace('_', '-'));
+                return true;
+            }
+            catch (CultureNotFoundException)
+            {
+                cultureInfo = null;
+                return false;
+            }
+        }
+
+        private static void OnConfigurationUpdated(object? sender, EventArgs e)
+        {
+            if (sender is IServerConfigurationManager configManager)
+            {
+                var uiCulture = configManager.Configuration.UICulture;
+                if (!string.IsNullOrEmpty(uiCulture))
+                {
+                    CultureInfo.DefaultThreadCurrentUICulture = new CultureInfo(uiCulture);
+                }
+            }
         }
 
         /// <summary>
@@ -420,6 +483,12 @@ namespace Emby.Server.Implementations.Localization
         /// <inheritdoc />
         public string GetLocalizedString(string phrase)
         {
+            return GetLocalizedString(phrase, CultureInfo.CurrentUICulture.Name);
+        }
+
+        /// <inheritdoc />
+        public string GetServerLocalizedString(string phrase)
+        {
             return GetLocalizedString(phrase, _configurationManager.Configuration.UICulture);
         }
 
@@ -436,6 +505,12 @@ namespace Emby.Server.Implementations.Localization
                 culture = DefaultCulture;
             }
 
+            // Normalize BCP-47 hyphenated codes to Jellyfin's underscore-based codes
+            if (_bcp47ToJellyfinMap.TryGetValue(culture, out var mapped))
+            {
+                culture = mapped;
+            }
+
             var dictionary = GetLocalizationDictionary(culture);
 
             if (dictionary.TryGetValue(phrase, out var value))
@@ -443,6 +518,15 @@ namespace Emby.Server.Implementations.Localization
                 return value;
             }
 
+            if (!string.Equals(culture, DefaultCulture, StringComparison.OrdinalIgnoreCase))
+            {
+                var fallback = GetLocalizationDictionary(DefaultCulture);
+                if (fallback.TryGetValue(phrase, out var fallbackValue))
+                {
+                    return fallbackValue;
+                }
+            }
+
             return phrase;
         }
 
@@ -450,26 +534,17 @@ namespace Emby.Server.Implementations.Localization
         {
             ArgumentException.ThrowIfNullOrEmpty(culture);
 
-            const string Prefix = "Core";
-
-            return _dictionaries.GetOrAdd(
+            return _cultureOnlyDictionaries.GetOrAdd(
                 culture,
-                static (key, localizationManager) => localizationManager.GetDictionary(Prefix, key, DefaultCulture + ".json").GetAwaiter().GetResult(),
-                this);
-        }
-
-        private async Task<Dictionary<string, string>> GetDictionary(string prefix, string culture, string baseFilename)
-        {
-            ArgumentException.ThrowIfNullOrEmpty(culture);
-
-            var dictionary = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
-
-            var namespaceName = GetType().Namespace + "." + prefix;
-
-            await CopyInto(dictionary, namespaceName + "." + baseFilename).ConfigureAwait(false);
-            await CopyInto(dictionary, namespaceName + "." + GetResourceFilename(culture)).ConfigureAwait(false);
+                static (key, localizationManager) =>
+                {
+                    var dictionary = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+                    var namespaceName = localizationManager.GetType().Namespace + ".Core";
+                    localizationManager.CopyInto(dictionary, namespaceName + "." + GetResourceFilename(key)).GetAwaiter().GetResult();
 
-            return dictionary;
+                    return dictionary;
+                },
+                this);
         }
 
         private async Task CopyInto(IDictionary<string, string> dictionary, string resourcePath)
@@ -508,77 +583,55 @@ namespace Emby.Server.Implementations.Localization
         /// <inheritdoc />
         public IEnumerable<LocalizationOption> GetLocalizationOptions()
         {
-            yield return new LocalizationOption("Afrikaans", "af");
-            yield return new LocalizationOption("العربية", "ar");
-            yield return new LocalizationOption("Беларуская", "be");
-            yield return new LocalizationOption("Български", "bg-BG");
-            yield return new LocalizationOption("বাংলা (বাংলাদেশ)", "bn");
-            yield return new LocalizationOption("Català", "ca");
-            yield return new LocalizationOption("Čeština", "cs");
-            yield return new LocalizationOption("Cymraeg", "cy");
-            yield return new LocalizationOption("Dansk", "da");
-            yield return new LocalizationOption("Deutsch", "de");
-            yield return new LocalizationOption("English (United Kingdom)", "en-GB");
-            yield return new LocalizationOption("English", "en-US");
-            yield return new LocalizationOption("Ελληνικά", "el");
-            yield return new LocalizationOption("Esperanto", "eo");
-            yield return new LocalizationOption("Español", "es");
-            yield return new LocalizationOption("Español americano", "es_419");
-            yield return new LocalizationOption("Español (Argentina)", "es-AR");
-            yield return new LocalizationOption("Español (Dominicana)", "es_DO");
-            yield return new LocalizationOption("Español (México)", "es-MX");
-            yield return new LocalizationOption("Eesti", "et");
-            yield return new LocalizationOption("Basque", "eu");
-            yield return new LocalizationOption("فارسی", "fa");
-            yield return new LocalizationOption("Suomi", "fi");
-            yield return new LocalizationOption("Filipino", "fil");
-            yield return new LocalizationOption("Français", "fr");
-            yield return new LocalizationOption("Français (Canada)", "fr-CA");
-            yield return new LocalizationOption("Galego", "gl");
-            yield return new LocalizationOption("Schwiizerdütsch", "gsw");
-            yield return new LocalizationOption("עִבְרִית", "he");
-            yield return new LocalizationOption("हिन्दी", "hi");
-            yield return new LocalizationOption("Hrvatski", "hr");
-            yield return new LocalizationOption("Magyar", "hu");
-            yield return new LocalizationOption("Bahasa Indonesia", "id");
-            yield return new LocalizationOption("Íslenska", "is");
-            yield return new LocalizationOption("Italiano", "it");
-            yield return new LocalizationOption("日本語", "ja");
-            yield return new LocalizationOption("Qazaqşa", "kk");
-            yield return new LocalizationOption("한국어", "ko");
-            yield return new LocalizationOption("Lietuvių", "lt");
-            yield return new LocalizationOption("Latviešu", "lv");
-            yield return new LocalizationOption("Македонски", "mk");
-            yield return new LocalizationOption("മലയാളം", "ml");
-            yield return new LocalizationOption("मराठी", "mr");
-            yield return new LocalizationOption("Bahasa Melayu", "ms");
-            yield return new LocalizationOption("Norsk bokmål", "nb");
-            yield return new LocalizationOption("नेपाली", "ne");
-            yield return new LocalizationOption("Nederlands", "nl");
-            yield return new LocalizationOption("Norsk nynorsk", "nn");
-            yield return new LocalizationOption("ਪੰਜਾਬੀ", "pa");
-            yield return new LocalizationOption("Polski", "pl");
-            yield return new LocalizationOption("Pirate", "pr");
-            yield return new LocalizationOption("Português", "pt");
-            yield return new LocalizationOption("Português (Brasil)", "pt-BR");
-            yield return new LocalizationOption("Português (Portugal)", "pt-PT");
-            yield return new LocalizationOption("Românește", "ro");
-            yield return new LocalizationOption("Русский", "ru");
-            yield return new LocalizationOption("Slovenčina", "sk");
-            yield return new LocalizationOption("Slovenščina", "sl-SI");
-            yield return new LocalizationOption("Shqip", "sq");
-            yield return new LocalizationOption("Српски", "sr");
-            yield return new LocalizationOption("Svenska", "sv");
-            yield return new LocalizationOption("தமிழ்", "ta");
-            yield return new LocalizationOption("తెలుగు", "te");
-            yield return new LocalizationOption("ภาษาไทย", "th");
-            yield return new LocalizationOption("Türkçe", "tr");
-            yield return new LocalizationOption("Українська", "uk");
-            yield return new LocalizationOption("اُردُو", "ur_PK");
-            yield return new LocalizationOption("Tiếng Việt", "vi");
-            yield return new LocalizationOption("汉语 (简体字)", "zh-CN");
-            yield return new LocalizationOption("漢語 (繁體字)", "zh-TW");
-            yield return new LocalizationOption("廣東話 (香港)", "zh-HK");
+            return _localizationOptions;
+        }
+
+        private static (IReadOnlyList<LocalizationOption> Options, FrozenDictionary<string, string> Bcp47ToJellyfinMap) BuildLocalizationData()
+        {
+            var options = new List<LocalizationOption>();
+            var bcp47Map = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+            var prefix = CoreResourcePrefix;
+
+            foreach (var resource in _assembly.GetManifestResourceNames())
+            {
+                if (!resource.StartsWith(prefix, StringComparison.Ordinal)
+                    || !resource.EndsWith(".json", StringComparison.Ordinal))
+                {
+                    continue;
+                }
+
+                // Extract culture code from resource name: "...Core.de.json" -> "de", "...Core.pt-BR.json" -> "pt-BR"
+                var code = resource[prefix.Length..^5];
+
+                // Record the BCP-47 → Jellyfin mapping for any resource file using underscores.
+                if (code.Contains('_', StringComparison.Ordinal))
+                {
+                    bcp47Map[code.Replace('_', '-')] = code;
+                }
+
+                // Skip the base language file — en-US is added explicitly below
+                if (code.Equals(DefaultCulture, StringComparison.OrdinalIgnoreCase))
+                {
+                    continue;
+                }
+
+                var displayName = GetDisplayName(code);
+                options.Add(new LocalizationOption(displayName, code));
+            }
+
+            // Ensure en-US is always present
+            options.Add(new LocalizationOption("English", DefaultCulture));
+
+            options.Sort((a, b) => string.Compare(a.Name, b.Name, StringComparison.OrdinalIgnoreCase));
+            return (options, bcp47Map.ToFrozenDictionary(StringComparer.OrdinalIgnoreCase));
+        }
+
+        private static string GetDisplayName(string cultureCode)
+        {
+            // Custom/novelty codes like "pr" (Pirate) — fall back to code itself
+            return TryGetCultureInfo(cultureCode, out var cultureInfo)
+                ? cultureInfo.NativeName
+                : cultureCode;
         }
 
         /// <inheritdoc />