As a bilingual person, I've had moments in which I've had to switch to the other language in order to have more appropriate words for the circumstances. "Oh, you know, it's called ________ but there's no English translation..." (see here for some untranslatables from different cultures)
There seems to be a varying distribution of words per subject, across different languages. (Also related to word populations, do more frequently-used words tend to be shorter?) There are many more dedicated words in Vietnamese to describe tastes, smells, and the way different nouns are carried, while there seem to be vastly more English words dedicated to medical conditions and legal processes. One of my college professors once observed that during periods of extreme excitement or extreme sadness, her bilingual students tended to gravitate toward their non-English language to express those emotions.
I'm finding that my personality as a Vietnamese speaker is completely different from my personality as an English speaker. We seem to speak faster and louder and with higher pitch, and with many more hand gestures. I find that I'm more affectionate and tactile and stand closer to others. Vietnamese is very songlike (tonal) and has an aquatic flow of one-syllable words. As a linguistic result, I trip over fewer sentences but overrun them more frequently – while (personally) using a lot more onomatopoeia, silly words, falsetto, and a more nasally laugh.
Aside from the practicalities of looking good on a résumé, it turns out there are evolutionary benefits to bilingualism and multilingualism such as in preventing or slowing the onset of Alzheimer's, thanks to how well it exercises the brain's neural pathways (Read more about this news of a recent study released May 2011, or its abstract).
Ellen Bialystok, a research professor involved in the aforementioned study, points out in an interview that bilingualism provides skills for a person to better differentiate between relevant and irrelevant information. The system within a brain that enables bilingualism is also the system "what makes it possible for you to hold two different things in your mind at one time and switch between them," she explains, although the benefit doesn't seem to apply to foreign language use that is only occasional.
And for many other reasons besides, bilingualism is good for child brain development. By the time most of us wish we had more languages under our belts, we've think we've bypassed the ideal age range for picking them up, even though some experts say there's no cutoff.
Many monolingual parents are taking measures anyway to prevent this regret for their children, and as a recent trend, many New York parents are hiring nannies who are then asked only to speak their native language on the job. Can these decisions have lasting impact? Children who stop speaking their native language early, such as by first or second grade, tend to lose their ability to speak it. The most pressing question for me is always to do with duty – how much duty do we have to preserve our cultural identities through language? Am I a more successful human being if I pass on my heritage to the next generation? I like to think of it as a gift I could someday give.