Just spelunking through the code (search the core stylesheet to see how the front end decides what language to use and then follow from there) I can see there is no documented user settable way to do this. You should file a suggestion with Wolfram Research to make this user settable here.
That being said, it looks like you can set something like
ExternalEvaluate`Private`$FElang = "Shell"
during a session and all subsequent external language cells will be of the "Shell" variety.
