You can already do that for subtitles embedded in your files or stored alongside your media as local assets.
Edit the show or season
Go to the advanced tab
Select/set the preferred subtitle for the entire show or season
Save your changes
This cannot be done with online subtitles as those can be all over the place.
This is already covered in existing suggestions. Therefore, please comment/vote in those threads in order to help avoid distracting or cannibalizing votes.