Right now, the html5 track specification has the attribute "default" that specifies which track should be on by default.
What would be better is my mind is to junk that attribute and instead have a user preference.
Someone who is deaf could set the preference to captions in their language, so that in a video with track information - first it would look for tracks with an kind attribute of captions and a srclang attribute of their native language, and automatically turn that track on. If such a track doesn't exist, then look for kind subtitles with their srclang and turn it on.
Someone who isn't deaf but wants subtitles could set their preferences to look in the opposite order.