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I've stopped recommending Plex to newcomers, because Jellyfin is ready for families now
(www.xda-developers.com)
Jellyfin is a free and open-source media server and suite of multimedia applications designed to organize, manage, and share digital media files to networked devices.
But I'm saying that adding subtitles is the Administrator's job anyways. If using the app on a phone to download and add subtitles is too complicated for the average user to do, why would the ability to do so on the AndroidTV version of the app be any different?
Users don't have to search for subtitles to download when they use DVD's or Blu-Rays, or when they are watching any commercial streaming service.