If you don’t have a current backup and can’t restart your old container, then I’d switch to the PG13 template and get a working setup, then make a backup, then set up a new server and restore the backup there. You can set it up with whatever language you like when it’s an empty database and then do the restore (and maybe the language will magically get converted?).
Moving to a new server ensures that you don’t crash your existing server.