Every good programmer should know at least a few programming languages. Why? Because some problems can be solved easily in one language and (nearly) impossible in the other. For managing big data and computation, I prefer C++. It is powerful as C, Pascal and other low level languages. When I need to make just a few system calls, I use bash or Python. I really like sys.os(). It is simple and fast enough. Also I use python for web development. Django is in my opinion far the best environment. Safe, powerful and easy to learn. You may need a project or two, to get all the necessary skills to code in django fluently. But then you may skip any obstacle costumers wants. If they want a very powerful backend (big data computation), you can always use c++ (or and other language) to make it.
And why not Java as it is so popular? Simply because of the sandbox and many compiler versions. I think Java was a shot in the dark. And why to make a class for literally everything? I tried Java and I hate it even more.