Fix zh-CN subtitle language display

The DisplayTitle property was using .NET's CultureInfo.GetCultures(NeutralCultures)
to resolve language display names. Since zh-CN is a specific culture (not neutral),
it would fall back to the base 'zh' code, resulting in generic 'Chinese' instead
of 'Chinese (Simplified)'.

This change adds a LocalizedLanguage property to MediaStream that gets populated
via LocalizationManager.FindLanguageInfo() when streams are retrieved from the
database. This leverages Jellyfin's existing iso6392.txt mappings which correctly
map zh-CN to 'Chinese (Simplified)'.

The same pattern is already used for other localized strings like LocalizedDefault
and LocalizedExternal.

1 files changed, 6 insertions, 0 deletions

diff --git a/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs b/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs
index 7eb13b7408..64874ccad7 100644
--- a/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs
+++ b/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs
@@ -158,6 +158,12 @@ public class MediaStreamRepository : IMediaStreamRepository
             dto.LocalizedDefault = _localization.GetLocalizedString("Default");
             dto.LocalizedExternal = _localization.GetLocalizedString("External");
 
+            if (!string.IsNullOrEmpty(dto.Language))
+            {
+                var culture = _localization.FindLanguageInfo(dto.Language);
+                dto.LocalizedLanguage = culture?.DisplayName;
+            }
+
             if (dto.Type is MediaStreamType.Subtitle)
             {
                 dto.LocalizedUndefined = _localization.GetLocalizedString("Undefined");