I know JS, however most of the projects I work on use Java or Python. I'm looking for a language that will give me a good understanding of the backend and will help me pick up other server side languages when I need to start on a new project.
In this case, my favorite back-end language is Scala, with the Play Framework. It has an excellent threading model, asynchronous processing, stateless model, a functional, immutable data programming style that fits well with multi-threading, great performance, and access to a vast array of Java libraries and tooling.