Plex will auto-pick forced subtitles if things are configured correctly. However, Plex remembers subtitle settings. So, if you ever select a different subtitle track, Plex will remember the choice the next time you play the movie. It will not automatically go back to the forced subtitle track.
Regarding configuring things, there are two parts: a) marking subtitles and b) system settings.
If using embedded subtitles, tag the subtitle track as forced. Also, make sure the language is set correctly. Set it to the language of the subtitle, not what is spoken onscreen. Use MKVToolNix or similar tools to set the tags.
If using external/sidecar subtitles, add “forced” to the subtitle name: movie_name (year).lang.forced.srt (ex: Avatar (2009).eng.forced.srt). The movie name and subtitle name must match, so if you’ve extra info in the movie name (resolution, codec, etc), include it in the subtitle name as well.
Next, configure your Plex account appropriately. I speak English, so I want the English translation for subtitles. Here’s my account settings (check “auto select” to make additional options appear):
