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.

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;
         }