(Warning: much of this comment is wild speculation.)

Pros:

• Knowing many languages is fucking awesome.
• You'll be able to consume media in the original language. (Translations exist but Sturgeon's Law applies.)
• Like TheOtherDave says, in some places not knowing the local language may mark you as an outsider. And in some other places (basically anywhere other than Northern Europe and capitals), if you don't know the local or national language it may be much harder than you might think to get by, whether or not you mind being seen as an outsider.
• Learning an n-th language now may make it much easier to learn an (n + 1)-st language later on (IOW, learning languages may itself be a learnable skill), especially if the two are related or otherwise similar; this may be useful if you think there's a large chance you'd need to learn a new language in the future but don't know which one yet.
• Knowing several languages which carve up thingspace in different ways may prevent certain failure modes.

Cons:

• Learning a language takes up time that you could use for something else. (But then again, so does watching TV or reading Cracked.com.)
• Certain people (namely, those who resent being made to learn a language in school which they haven't ever since had any practical use for) will think of you as a geek and dislike you.