French. Or Arabic. Or whatever you don't already speak. Snark aside, the standard answer is something like "Learn a language in each of the different kinds or

French. Or Arabic. Or whatever you don't already speak. Snark aside, the standard answer is something like "Learn a language in each of the different kinds or programming (functional, procedural, etc)". Really, though, programming is not about a particular syntax. It's about learning how to think and deconstruct problems. So that brings me back to the spoken languages. Learn new things that will challenge you (spoken languages are great), and learn how to deconstruct problems. And above all, have fun doing it.