Translation agency American Translation Partners has developed a nifty language locator that allows you to select a country and find out which language or languages are spoken there along with a few other details, as shown here:
Sources for the data include the Ethnologue, CIA World Factbook, and encyclopedias.
Try it our yourself.
This is an interesting interface. I’ve dug around into this problem before on my own Hacklog: blog. The only complaint I would have is that it sometimes takes several clicks in the country select box (once you’ve chosen a continent) to find a country. Why not just make the whole country list visible and clickable?