Ask HN: Interface Engineer Deciding Which System Language To Learn

I have focused the majority of my web development career on developing web interfaces with Javascript/CSS/HTML. I'm pretty comfortable on the command line in a Linux environment. Last night I started playing the AWS EC2 and I'm itching to start building a dynamic application. The last piece is picking a language, the problem, there are so many I don't know which one to invest my time in.

Right now I'm considering:

PHP Python Ruby on Rails Node.js

Any insight is greatly appreciated. Thanks!