I know this question was here a lot of times and can't be answered at all, but im not looking for a single name, but rather for an advice in my situation.

I learned programming with Java and now I'm developing in Java for more or less 5 years (at the university) and I thinks my programming skills their are really ok/average. I have also small experience in C/C++ and C#. Now I have some spare time and I'd like to learn a new language or deepen the knowledge of Java/C/C++. But how to choose the right language to learn? I'd like to learn a language which will be usefull in the future concerning working in a software development business? I know there is no single answer, but I'm sure you could mention some languages that are more usefull than others.