Speak English.

Gregg Easterbrook writes in TNR that it's great when languages die out. [ADDED: The link is for subscribers only -- sorry.] The fewer languages the better. There's no interest comparable to the interest in biodiversity. It's better for more people to be able to communicate.

[T]he fewer languages, the better--and I say this not just because English may win the competition. Mandarin might win; Barbara Wallraff of The Atlantic Monthly devoted a lengthy 2001 story to the idea that Mandarin will best English in the struggle to be the global tongue. Okay, maybe Mandarin comes out on top. As long as the number of languages in common usage keeps declining, I'll be happy.

Well, I can't remember reading Wallraff's article, but I can't imagine what mechanism would drive the spread of Mandarin much beyond China. Easterbrook himself notes the recent NYT article about the drive to teach English in Mongolia:

Even here on the edge of the nation's capital, in this settlement of dirt tracks, plank shanties and the circular felt yurts of herdsmen, the sounds of English can be heard from the youngest of students - part of a nationwide drive to make it the primary foreign language learned in Mongolia, a landlocked expanse of open steppe sandwiched between Russia and China. "We are looking at Singapore as a model," Tsakhia Elbegdorj, Mongolia's prime minister, said in an interview, his own American English honed in graduate school at Harvard. "We see English not only as a way of communicating, but as a way of opening windows on the wider world."

Easterbrook:

Why is Singapore so much more affluent than its neighbors? In part owing to a long-standing policy of teaching public-school students English. Someone safely tenured in a comfortable Western university might idealize living a subsistence lifestyle speaking a rare language unintelligible except to one's tribe. For citizens of the developing world, speaking a top ten language opens doors to a better life.

If you were making policy in one of those countries, choosing a second language to teach everyone, wouldn't you, like Mongolia, pick English?

UPDATE: From Ireland, Paul Musgrave disagrees and has a lot to say, including a lot about Irish.