I'm going to make a rather bold statement:
There should be only one language.
Honestly, why the redundancy? It's not as though, if English were to "crash," we'd have Italian waiting on standby to pick up its slack as part of some array of languages. The less that we are able to understand one another, the worse off we are.
We can't solve the problem by becoming multilingual, either, because:
1. It'd be pretty difficult to learn every language on Earth.
2. If I know English and Spanish and you know English and Spanish, that's two people who each have two distinct symbols in their brains for every imaginable human conception. Multiply this waste of time and space by six billion and you'll see where I'm going with this. You should only learn a second language if you need to in order to understand someone who doesn't already know your language -- but in an ideal society, this problem wouldn't exist in the first place.
Imagine having two cars, but living alone. Imagine owning two pairs of shoes when you only need one to protect your feet. Imagine having two computer keyboards that you occasionally swap back and forth for fun. Imagine having two beds to sleep in and oscillating between them at random.
A duplicate item needn't be identical to the original in order to qualify as being functionally void or needlessly redundant. Sure, you may enjoy the aesthetic variation, the novelty, the sheer variety; perhaps these qualities supersede boredom. There's nothing wrong with this, but if you're acquiring duplicate items at the expense of something more materially valuable at that moment, then you're woefully ignoring opportunity cost, which necessarily leads to your generating wasted space.
Brain space is no different from other forms of space; it's certainly finite, above all else. Don't waste time learning a new version of something with which you're already familiar when there's much more to be learned in its place; doing otherwise promotes pretentiousness, frivolous socialization, and, ultimately, a fragmented species intermittently predominated by huge communications holes.
Important disclaimer: If you're not American, British, Australian, or Canadian and you read this blog, you're likely bilingual. Please take note that it isn't your fault that a second language has been imposed upon you by academia; furthermore, considering the gradual encroachment of the English language upon much of the territory of the other languages of the world, there may be some practical benefit in your knowing English. Just keep in mind that it'd be really dumb of you to decide to learn Arabic for fun or to show off how cultured you are to friends. In any case, this post isn't about what you should be doing with your own personal life, but what a society as built from scratch should look like.