AFAIK the best way to get subs to display in Plex is has the media and the sub the sane filename
Ex:
name.of.tv.episode.s01.e01.avi
name.of.tv.episode.s01.e01.srt
I even checked to make sure there wasn’t an issue with the subs and loaded it up in MPC-HC and all was well. So what gives?
name.of.tv.episode.s01e01.avi (remove the dot between s01 and e01)
name.of.tv.episode.s01e01.en.srt (en tells Plex it is an english subtitle file)
If that does not work, please attach a subtitle file that does not work for you and I will test in my setup.
I’ve been using Plex for years I wonder why some subs won’t load until you add a language. I just had gotten used to “unknown srt” when selecting the sub. Thank you.
My guess is that the language tag was not the thing that solved this. I would say that the removal of the dot solved it 
