None. I've got enough of them. What I need to get better at is maths, signal processing, and graphics. Chiming in to reject the idea that we should spend our careers churning through languages.
Learning a new language can also be a supplement, say learning haskell in such a way that you improve your math ability and critical thinking at the same time. Or attempting to write a project in a new language would be far more interesting say if you are burned out from a job in a certain programming language. New and different things can be very refreshing!