It is really tedious to select subtitles by episode. I have shows in languages I understand and also in some I don’t. Also some shows - with languages I understand - I typically watch with others who prefer to have subtitles.
It would be brilliant to be able to set subtitle preference by show.
Also it would be useful to have the subtitle option “unkown.srt” available as a selection. Some shows do have subtitles but not named in a way that Plex understands. There is typically just the one subtitle file named the same as the video file with the extension “srt” but without a language designation. If subtitles would be opted “on” then it would use that file.
I’m not sure if this is correct in my understanding, but it sounds like you want to see certain subtitles on specific episodes? If so, I watch a ton of Anime. When I was ripping my discs, I check to see which subtitle tracks had the ones where just the Signs and minor stuff appear, instead of the entire episode. When using MakeMKV, I chose the appropriate subtitle and changed it to df (which make it default and forced). This makes it a permanent placement in the MKV file and will show up as say: English Forced (PGS). Keep in mind that you do this before the video file is created, but you can do this afterwards, but it creates a separate file once again.
You can use MKVToolNix and change the flags when in the Header Editor section. Under subtitles, select the one you need to be forced and select “yes” for these 3 things (for flags): Default track - Forced Display & Track Eneabled. In some cases for any of those 3, you may have to select Add element, before selecting yes. Hope that helps and I can help you as well. Using this method also just updates your current file. So, your existing file will be overwritten, not become a separate file like MakeMKV.