I found, in terms of learning a language quickly, that taking a course in programming language concepts where you have to learn a variety of different languages

I found, in terms of learning a language quickly, that taking a course in programming language concepts where you have to learn a variety of different languages, covering functional, procedural, OO, symbolic, static and dynamically typed, etc, makes picking up new languages easier. Most programming languages use ideas that already exist in other languages, so the more languages you learn, provided they cover the breadth of the major paradigms, the easier it is to pick up new ones. Alternatively, if you can't take a course or don't want to, make the effort to become competent in a number of languages that cover the spectrum described above. C++ developer? Learn python for the dynamic \ functional aspects. Then learn Haskel to force you to think pure functional. Or OCaml. Learning js, due to the way it implements objects, also taught me a lot about object oriented programming, as you are forced to use closures to get private variables, and it's object model is primitive so learning to work with that can be educational.