Fix localization lookup

Emby.Server.Implementations/Localization/Core/en_US.json

@@ -1,64 +0,0 @@
-{
-    "AppDeviceValues": "App: {0}, Device: {1}",
-    "Artists": "Artists",
-    "AuthenticationSucceededWithUserName": "{0} successfully authenticated",
-    "Books": "Books",
-    "ChapterNameValue": "Chapter {0}",
-    "Collections": "Collections",
-    "Default": "Default",
-    "External": "External",
-    "FailedLoginAttemptWithUserName": "Failed login attempt from {0}",
-    "Favorites": "Favorites",
-    "Folders": "Folders",
-    "Forced": "Forced",
-    "Genres": "Genres",
-    "HeaderContinueWatching": "Continue Watching",
-    "HeaderFavoriteEpisodes": "Favorite Episodes",
-    "HeaderFavoriteShows": "Favorite Shows",
-    "HeaderLiveTV": "Live TV",
-    "HeaderNextUp": "Next Up",
-    "HearingImpaired": "Hearing Impaired",
-    "HomeVideos": "Home Videos",
-    "Inherit": "Inherit",
-    "LabelIpAddressValue": "IP address: {0}",
-    "LabelRunningTimeValue": "Running time: {0}",
-    "Latest": "Latest",
-    "LyricDownloadFailureFromForItem": "Lyrics failed to download from {0} for {1}",
-    "MixedContent": "Mixed content",
-    "Movies": "Movies",
-    "Music": "Music",
-    "MusicVideos": "Music Videos",
-    "NameInstallFailed": "{0} installation failed",
-    "NameSeasonNumber": "Season {0}",
-    "NameSeasonUnknown": "Season Unknown",
-    "NewVersionIsAvailable": "A new version of Jellyfin Server is available for download.",
-    "NotificationOptionApplicationUpdateAvailable": "Application update available",
-    "NotificationOptionApplicationUpdateInstalled": "Application update installed",
-    "NotificationOptionAudioPlayback": "Audio playback started",
-    "NotificationOptionAudioPlaybackStopped": "Audio playback stopped",
-    "NotificationOptionCameraImageUploaded": "Camera image uploaded",
-    "NotificationOptionInstallationFailed": "Installation failure",
-    "NotificationOptionNewLibraryContent": "New content added",
-    "NotificationOptionPluginError": "Plugin failure",
-    "NotificationOptionPluginInstalled": "Plugin installed",
-    "NotificationOptionPluginUninstalled": "Plugin uninstalled",
-    "NotificationOptionPluginUpdateInstalled": "Plugin update installed",
-    "NotificationOptionServerRestartRequired": "Server restart required",
-    "NotificationOptionTaskFailed": "Scheduled task failure",
-    "NotificationOptionUserLockedOut": "User locked out",
-    "NotificationOptionVideoPlayback": "Video playback started",
-    "NotificationOptionVideoPlaybackStopped": "Video playback stopped",
-    "Original": "Original",
-    "Photos": "Photos",
-    "PluginInstalledWithName": "{0} was installed",
-    "PluginUninstalledWithName": "{0} was uninstalled",
-    "PluginUpdatedWithName": "{0} was updated",
-    "ScheduledTaskFailedWithName": "{0} failed",
-    "Shows": "Shows",
-    "StartupEmbyServerIsLoading": "Jellyfin Server is loading. Please try again shortly.",
-    "SubtitleDownloadFailureFromForItem": "Subtitles failed to download from {0} to {1}",
-    "TvShows": "TV Shows",
-    "Undefined": "Undefined",
-    "UserCreatedWithName": "User {0} has been created",
-    "UserDeletedWithName": "User {0} has been deleted"
-}

Emby.Server.Implementations/Localization/LocalizationManager.cs

@@ -566,11 +566,15 @@ namespace Emby.Server.Implementations.Localization
 
         private static string GetResourceFilename(string culture)
         {
-            var parts = culture.Split('-');
+            // Region codes may use a '-' (BCP-47, e.g. "pt-BR") or '_' (e.g. "es_419", "ar_SA") separator.
+            // Normalize the casing (lower-case language, upper-case region) while preserving the separator
+            // so the result matches the embedded resource file name, which is case-sensitive.
+            var separatorIndex = culture.IndexOfAny(['-', '_']);
 
-            if (parts.Length == 2)
+            if (separatorIndex > 0)
             {
-                culture = parts[0].ToLowerInvariant() + "-" + parts[1].ToUpperInvariant();
+                var separator = culture[separatorIndex];
+                culture = culture[..separatorIndex].ToLowerInvariant() + separator + culture[(separatorIndex + 1)..].ToUpperInvariant();
             }
             else
             {