Ultimately you're going to need to know multiple programming languages in your toolbox to be considered dangerous. I recommend Java, C++, Python and Javascript IMO.
I meant dangerous in a bad way. :P I agree though. I have some background in Python and I am learning JavaScript now. C++ is next.
The first language is 90% of the effort, it's not super unusual to be proficient in a new language in six months (for a given value of proficient). While obviously there are new concepts (static typing, functional programming), the first language is definitely the hardest.