Toward a World Language
To those anti-immigrant readers who might take this story as a warning about things to come: Don't worry. The Mankiw family is fully assimilated now, thank you very much. Sadly, however, I inherited this grandmother's language-learning ability (rather than that of my maternal grandmother, who mastered several languages seemingly without much effort). The only language I ever managed to become proficient at besides English was Fortran.
Austan points out, no doubt correctly, that there are tremendous incentives for people to learn the prevailing language (making it pointless and a bit mean-spirited to make learning English a legal requirement). In our increasingly globalized world, I wonder how fast the trend toward a common world language will be. I also wonder which language it will be. Although a list of the most spoken languages puts Chinese at about twice English, I would bet on English. If people are weighted by their income, English would probably be the number-one language by a large margin. (If anyone has seen that calculation, please let me know.) As incentives go to learn other languages, the number of dollars will likely exert a more powerful influence than the number of people.
Here's my prediction: In 500 years, English will be the world language. You can hold me to it.