I've studied SDL Trados Studio 2017 Enterprise Deployment Guide and currently found no 'settings' way to limit language list available for Pro/Workgroup license end user.
Meanwhile that might be really handy for a corporation (or even a Freelancer) since scrolling through this list is quite timeconsuming.
I know about favourite languages list at the top and altering source / target but i'm speaking about fresh Studio installation ready for jump start.
Introducing this option in future builds might really be beneficial.