I'm of the opinion that more people should know at least one other language. I'm not talking about some kind of cheesy tourist phrase-book or a I'm-trying-to-pick-up-girls-at-a-summer-course kind of thing.
I'm talking about honest-to-god bilingualism. I'm talking about being in a place where you have to use the language just to survive. I'm talking getting the power cut-off if you can't explain to the guy on the phone that your papers are in actual order. I'm talking about being able to order the food that you actually wanted. It's hard, it's difficult, and it'll turn your world upside down.
Being able to talk in another language means that you'll end up thinking differently. It's a strange thing to slip in and out of one language. Strange associations jump out at you from different angles, as you begin to understand different idioms. Each language has its favorite phrases and pronouns. By choosing that slightly different alternative to say the same thing - you start seeing the world in slightly different ways, with profound effects on what you easily see, or don't see. Speaking in another language, textures your world much more than a character mod in a DOOM extension pack.
There's also nothing quite like the loneliness and frustration in being stuck in a land where no one can communicate with you. People who are generally smart will feel the pain of being slow and retarded. Your tongue will feel thick and unwieldy. Whereas once you were always quick with a witty rejoinder, you are suddenly reduced to monosyllabic responses. Yet there are subtle joys. As you feel your language skills click up a gear, you will feel the pleasure of rediscovering latent social skills.
But more importantly, once you realize that, what was once your monolithic world, is but one of many worlds bound by your native vocabulary, you will realize that it is just that - only one world in a vast universe of possible worlds.