Many people say that they've forgotten a language they knew sometime earlier in their lives. One of my friends who lived for three years in Japan when she was young (preschool & kindergarten age) says she doesn't understand a word of Japanese now. We often forget second languages if we don't constantly practice them (like my French). However, having learned a language at an early age seems to work to one's advantage when learning that language again. It's easier to pick up the language, and one is likely to have a more accurate pronunciation.
Thankfully, I didn't really lose English when I moved to Korea, but my English did get rusty. I still have trouble conjuring up words sometimes because I think of them in Korean. I get nervous about making a mistake, which makes it more difficult to say what I want to say. However, it's the same the other way around. The other day, I was talking to my Korean friend on the phone and I couldn't remember the Korean word for raccoon for some reason. That's why I prefer Konglish, as I've said before. Anyway, getting back on track of the topic, so I was curious... Can we really "lose" a language?
I don't know how reliable this source is, but apparently, there was recently a French study that looked at Korean-born children who were adopted by French parents and thus, were no longer exposed to Korean. The key finding was that these individuals, when tested in their 20s and 30s, could not distinguish between Korean and Japanese. The conclusion was that learning a second language in childhood is in vain if exposure to that language isn't sustained, which seems correct based on our experiences.
There was also a study conducted by psychologist Benjamin Levy and his colleague Dr. Michael Anderson at the University of Oregon that found something called "first language attrition." The subjects of the study were native English speakers who had learned about a year of Spanish. They were told to repeat a word for a certain object in Spanish numerous times. After that, they were told to recall the word for the object in English. The researchers found that people had a more difficult time retrieving the English word after such Spanish exercises. The conclusion is that learning a second language can negatively affect your first language ability. You don't actually forget the word, but it is hard to roll it out on your tongue. So, vice versa, it's usually hard to learn a new language because our first language inhibits us from thinking in the other language. We have to temporarily "forget" our first language in order to learn the second language more quickly.
However, this research didn't really answer my question because it talked about short-term loss of language. I couldn't really find substantial research that would prove that we really can lose our languages, but it seems safe to conclude so.
I don't know if I just couldn't find the right study on the internet, but I feel this is an interesting topic worth researching. Some things I would want to know more about are: how much being exposed to a language at the developmental ages helps in acquiring that language later on in life, which aspect of language benefits most (because on personal intuition, I feel that having once spoken a language at least leaves the memory of the pronunciation, so when you learn it later on, you still have the capacity to produce the distinctive sounds in that language), how many languages a normal child can acquire and keep, etc.