If you immerse yourself in the early stages of learning a new language, you might start to lose your grasp on the old one for awhile; this phenomenon has the depressing name of “first language attrition”. Researchers from Spain and the US investigated what’s going on when this happens, and concluded that when people focus on words in the new language, they appear to be actively suppressing the memory of the old familiar words in their first language. This effect goes away when people become fluent in the second language (so it’s not a depressing zero-sum sort of thing, as it appears at first); perhaps it’s a useful brain strategy that helps cement the new language more firmly in memory by ignoring the old one temporarily.
You can read this article from Science Daily to learn more. I was amused at the intro to the Science Daily piece, which mentions one of the researchers musing about how it could be “possible to forget, even momentarily, words used fluently throughout one’s life.” Sadly enough it happens to me all the time, and not in the context of learning a second language either.
When you (or the authors of that article) talk about ‘first language’, is there any assumption on how long that language has been spoken by the person before they attempt to learn a new language? I ask, because I’ve always been fascinated by how different my cousins (who had more time in daycare and little kids) and my niece who spends a lot of time with her mom (and granny at times) are … w.r.t to remembering their mother-tongue (tamil). The elder of the cousins can actually say things in tamil now but hardly does (she spent a bit more time with her mom as a baby); the younger cousin pretty much cannot speak tamil; but my niece, who is already 7 (or close), still can speak fluent tamil though she’s been exposed to a lot of english (or american, I should say) speaking friends/acquantances. -AG
I think in this study it was adults learning a second language; I’m assuming they’d been speaking their first language for at least 18 years. Coincidentally I ran across something the other day about language learning in internationally adopted children (http://www.eurekalert.org/pub_releases/2007-01/afps-iac011807.php). This isn’t the same situation as in your family, because these children begin learning one language and then, while they are still young children, they have to start over and learn another language and give up their first one when they are adopted (they go through the same stages as children learning their first language, but quicker). But it’s interesting anyway.