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.

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