There’s already an existing suggestion discussing an option to support more preferred languages with regards to audio/subtitle selection. I suggest you comment/vote in that thread in order to help us avoid distracting or cannibalizing votes.
The part about showing subtitles for foreign parts is trickier. The different subtitle frames/segments aren’t tagged with a language. If you’re i.e. watching Quentin Tarantino’s „Inglorious Basterds“ you’ll have English, French and German audio. The forced English subtitle track will contain the frames/segments for the French and German dialogs. Plex has no means to know which is which if you e.g. speak English and French but no German… if the English forced subtitle is selected, it’ll always show the subtitles for both other languages.
