Fix zh-CN subtitle language display (#15947)

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.

3 files changed, 55 insertions, 46 deletions

diff --git a/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs b/Jellyfin.Server.Implementations/Item/MediaStreamRepository.cs
index 7eb13b740..64874ccad 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");
diff --git a/MediaBrowser.Model/Entities/MediaStream.cs b/MediaBrowser.Model/Entities/MediaStream.cs
index b1626e2c9..c443af32c 100644
--- a/MediaBrowser.Model/Entities/MediaStream.cs
+++ b/MediaBrowser.Model/Entities/MediaStream.cs
@@ -260,6 +260,8 @@ namespace MediaBrowser.Model.Entities
 
         public string LocalizedHearingImpaired { get; set; }
 
+        public string LocalizedLanguage { get; set; }
+
         public string DisplayTitle
         {
             get
@@ -273,29 +275,8 @@ namespace MediaBrowser.Model.Entities
                         // Do not display the language code in display titles if unset or set to a special code. Show it in all other cases (possibly expanded).
                         if (!string.IsNullOrEmpty(Language) && !_specialCodes.Contains(Language, StringComparison.OrdinalIgnoreCase))
                         {
-                            // Get full language string i.e. eng -> English, zh-Hans -> Chinese (Simplified).
-                            var cultures = CultureInfo.GetCultures(CultureTypes.NeutralCultures);
-                            CultureInfo match = null;
-                            if (Language.Contains('-', StringComparison.OrdinalIgnoreCase))
-                            {
-                                match = cultures.FirstOrDefault(r =>
-                                    r.Name.Equals(Language, StringComparison.OrdinalIgnoreCase));
-
-                                if (match is null)
-                                {
-                                    string baseLang = Language.AsSpan().LeftPart('-').ToString();
-                                    match = cultures.FirstOrDefault(r =>
-                                        r.TwoLetterISOLanguageName.Equals(baseLang, StringComparison.OrdinalIgnoreCase));
-                                }
-                            }
-                            else
-                            {
-                                match = cultures.FirstOrDefault(r =>
-                                    r.ThreeLetterISOLanguageName.Equals(Language, StringComparison.OrdinalIgnoreCase));
-                            }
-
-                            string fullLanguage = match?.DisplayName;
-                            attributes.Add(StringHelper.FirstToUpper(fullLanguage ?? Language));
+                            // Use pre-resolved localized language name, falling back to raw language code.
+                            attributes.Add(StringHelper.FirstToUpper(LocalizedLanguage ?? Language));
                         }
 
                         if (!string.IsNullOrEmpty(Profile) && !string.Equals(Profile, "lc", StringComparison.OrdinalIgnoreCase))
@@ -393,29 +374,8 @@ namespace MediaBrowser.Model.Entities
 
                         if (!string.IsNullOrEmpty(Language))
                         {
-                            // Get full language string i.e. eng -> English, zh-Hans -> Chinese (Simplified).
-                            var cultures = CultureInfo.GetCultures(CultureTypes.NeutralCultures);
-                            CultureInfo match = null;
-                            if (Language.Contains('-', StringComparison.OrdinalIgnoreCase))
-                            {
-                                match = cultures.FirstOrDefault(r =>
-                                    r.Name.Equals(Language, StringComparison.OrdinalIgnoreCase));
-
-                                if (match is null)
-                                {
-                                    string baseLang = Language.AsSpan().LeftPart('-').ToString();
-                                    match = cultures.FirstOrDefault(r =>
-                                        r.TwoLetterISOLanguageName.Equals(baseLang, StringComparison.OrdinalIgnoreCase));
-                                }
-                            }
-                            else
-                            {
-                                match = cultures.FirstOrDefault(r =>
-                                    r.ThreeLetterISOLanguageName.Equals(Language, StringComparison.OrdinalIgnoreCase));
-                            }
-
-                            string fullLanguage = match?.DisplayName;
-                            attributes.Add(StringHelper.FirstToUpper(fullLanguage ?? Language));
+                            // Use pre-resolved localized language name, falling back to raw language code.
+                            attributes.Add(StringHelper.FirstToUpper(LocalizedLanguage ?? Language));
                         }
                         else
                         {
diff --git a/tests/Jellyfin.Model.Tests/Entities/MediaStreamTests.cs b/tests/Jellyfin.Model.Tests/Entities/MediaStreamTests.cs
index f4c0d9fe8..c1a3a4544 100644
--- a/tests/Jellyfin.Model.Tests/Entities/MediaStreamTests.cs
+++ b/tests/Jellyfin.Model.Tests/Entities/MediaStreamTests.cs
@@ -108,6 +108,49 @@ namespace Jellyfin.Model.Tests.Entities
                     IsExternal = true
                 });
 
+            // Test LocalizedLanguage is used when set (fixes zh-CN display issue #15935)
+            data.Add(
+                "Chinese (Simplified) - SRT",
+                new MediaStream
+                {
+                    Type = MediaStreamType.Subtitle,
+                    Title = null,
+                    Language = "zh-CN",
+                    LocalizedLanguage = "Chinese (Simplified)",
+                    IsForced = false,
+                    IsDefault = false,
+                    Codec = "SRT"
+                });
+
+            // Test LocalizedLanguage for audio streams
+            data.Add(
+                "Japanese - AAC - Stereo",
+                new MediaStream
+                {
+                    Type = MediaStreamType.Audio,
+                    Title = null,
+                    Language = "jpn",
+                    LocalizedLanguage = "Japanese",
+                    IsForced = false,
+                    IsDefault = false,
+                    Codec = "AAC",
+                    ChannelLayout = "stereo"
+                });
+
+            // Test fallback to Language when LocalizedLanguage is null
+            data.Add(
+                "Eng - ASS",
+                new MediaStream
+                {
+                    Type = MediaStreamType.Subtitle,
+                    Title = null,
+                    Language = "eng",
+                    LocalizedLanguage = null,
+                    IsForced = false,
+                    IsDefault = false,
+                    Codec = "ASS"
+                });
+
             return data;
         }