Don't get me wrong, you can get to be a damn good Blub programmer. You can also work at an international company, headquartered in US, without being skilled enough at English to appreciate Shakespeare's work; or, for that matter, loathe it, since you need the same level of proficiency.
I do believe that you'll never get to enjoy English to the fullest, if you don't explore its poetic reaches. In words of Wilhelm von Humboldt:
The diversity of languages is not a diversity of signs and sounds but a diversity of views of the world.
I believe that the same applies to programming languages. Yes, you can be really good with just Java. But just by learning C# on the side, you can gain valuable insights without stepping too far from your comfort zone. Learning Lisp, Self, Python and Factor, to name a few, will give you whole new worldviews, make you think differently. You don't really need that stuff to make software (or money). But it can make you a hell of a lot better at it and, more importantly, it can make you enjoy it more.