I’ll be keeping an eye on this thread.
I attempted to play with subtitles and was only somewhat successful. The handbrake CLI does have a command to set the default lang. It will grab all the english tracks, and only the english tracks while discarding the rest. If the tracks are unmarked (und) it will grab all the tracks you specify (–subtitle “1,2,3,4,5”).
It works well, but is buggy, at least with the handbrake cli. The problem is when your file has PGS subs. Handbrake doesn’t handle them properly and always wants to either burn them right into the file (if output is mp4), or set them as forced (for mkv). Burned is a big no-no for me, and forced is a nuisance that needs to constantly be turned off in plex per each file.
I eventually gave up and just decide the pass the first 5 subs. English is always first or second, and is always within the first 5 etc, so it works. Not elegant, or efficient, but works 