On Windows, the issue has already been manageable for a long time. Just don’'t use the Web app, but either Plex HTPC or Plex for Windows. Customizing Subtitle appearance in Plex Player for Windows/Mac/Linux and Plex HTPC and PMP
Thou shan’t have any other text encodings than UTF-8.
Unicode is a thing. It’s been supported in players across all platforms for decades now. There is no good reason to keep your files in antiquated 8bit encodings.
