Either Javascript, since it's the language of the web on the client side, and it's fairly easy to get started with, or a very different language, like Haskell o

Either Javascript, since it's the language of the web on the client side, and it's fairly easy to get started with, or a very different language, like Haskell or Erlang. The advantage of learning something very different is that it will give you a much better overview, and if you ever need or want to learn yet another language, it will be that much easier.