IMHO it's irrelevant. If you can code in Java you can code is most relevant modern languages.
A loop is a loop is a loop, the syntax may very slightly from language to language but basically they all boil it down to start here (a), go here (z), and optionally increment by something (m), stop when here (z) is reached or exceeded (or whatever necessary condition is met).
The nitty gritty differences in inheritance, polymorphism, garbage collection, syntax etc can be picked up really quickly once you realize that it's almost all the same.
No single language is going to make or break your pen testing career, the thing that's important is understanding how to design things logically and robustly.