Of course, it’s very rational (in fact, often unavoidable) to learn a language when you have no choice—if you live or work where that language is all that’s spoken. That’s part of the beauty of exchange programs and cross-cultural romance. But beyond that, you’re better off hiring a translator or interpreter when you need one: it’s much cheaper, and likely to be just as effective.
That’s why Americans rarely learn foreign languages: the benefits simply aren’t there. That’s also why children learn languages “more easily” than adults: they have much less choice in the matter, especially if their peers are speaking a different language. Adult professionals, like the aforementioned lawyers in Tokyo, can just outsource their language needs, and save a lot of time and money by doing so.
So what accounts for those of us who learn languages for the hell of it (including the authors of this blog)? Basically, we’re nuts. Not thinking things through. I love knowing Japanese and I keep learning more, but it hasn’t been particularly useful in my life, except when haggling with electronics dealers in Den-Den Town. Yet I keep learning it, probably because the amenities in Japan are so tempting, and, like most Westerners who know Asian languages, I have way too much fun flaunting the skills (what Jay Rubin calls the “look, Mom! I’m reading Japanese!” effect).
On the other hand, a professor of mine, who loves Japan enough to spend a few months teaching there every other year or so, has stopped bothering with language classes. For him, there isn’t much necessity: his family speaks English, his classes are taught in English, and a couple of lines from a phrasebook are all he needs to order lunch or locate an English speaker. Maybe if he were dropped into a high school classroom in Osaka, he would start figuring out those characters.
One of my favorite analyses on this issue comes from amateur linguist/online ne’er-do-well Mark Rosenfelder, who wrote a nice, meaty article on the “how”s and “why”s of language acquisition, and reached the same conclusion: language learning almost always comes from necessity, and the exceptions can be counted as strange obsession. So before you go off to learn a language for kicks, consider what your obsession is.