| Simply because if you're taking a foreign language class in your native language, you simply leave that class, and bam! it's back to being surrounded with your primary language. The new language doesn't stick as much if you're thinking primarily in your native language.
However, if you were to go to a foreign country, it goes from being a matter of academic interest to a matter of survival. For instance, I was raised in a tri-lingual household (we all spoke English but my father taught me Spanish and my mother taught me French), and I went to Spain with a friend of mine who grew up speaking only English. Now, he had studied Spanish for about 6 years, but by the time we got there, it was as if none of that had any use, because he hadn't been completely exposed to the language. He would take class, and then go home and speak English. However, after one week of being in Spain, he could speak it near fluently (he could do all right before we left, but now he can speak it without problems). This is simply because your brain makes a major effort to really learn the language, simply because it becomes a matter of survival.  |