Traveling abroad presents an ideal opportunity to master a foreign language. While the immersion process facilitates communication in a diverse world, people are often surprised to find they have difficulty returning to their native language.  
 
This phenomenon is referred to as first-language attrition and has University of Oregon psychologist Benjamin Levy wondering how it is possible to forget, even momentarily, words used fluently throughout one's life.
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jinx: You don't even have to be learning another. Simply ceasing to speak your own for extended periods starts to do it to you.
yep
Kassi42: This is definitely true when you learn to type in a different layout. I can't type in QWERTY to save my life. It feels like I'm stuck in a spiderweb.
maybe..
pneum0nic: when i immersed myself in russian culture by living in st petersburg for 1 yr, i started having dreams in russian. i apparently was talking in my sleep!