You may want to set the language of your audio while you are modifying the files. This will help with displaying subtitles when needed(if set correctly).
track:a1 --set flag-default=1 --set flag-forced=0 --set language=eng
This assumes english(eng).
Though I strongly suggest you use caution with the command --delete name on tracks cause you could be removing detailed descriptions of the tracks. Things like a “commentary track” or “Subtitles - SDH”, or similar.
More on what I am trying to say.
While Plex doesn’t support this(yet). I hope someday it will.