Java isn't particularly great for game development; but it can be used. I've done some work in it; performance wasn't fantastic but it was usable for simple games.
I don't suggest Python for game dev at all.
However.
C# is a fantastic language to know if you do game development. A lot of impressive indie games have been written in it, and if you're not working in a large team, the time you'll save using C#/XNA over C++ adds up very fast.
That being said, it's important to be good with C++.
What I suggest is that you do familiarize yourself with C++, but primarily write portfolio piece games in C# (as you'll find you actually finish them). At the end of the day, having a strong portfolio is going to say a lot more for your programming skills on an application than anything else. So yes, know C++, but don't fall into the trap of feeling confined to it.