I’ve read that at a very early age, our brains can understand the sounds of pretty much any language spoken fairly easily, but as our language skills develop we lose this ability and concentrate only on the sounds in the language we learn to speak. This is why folks who speak certain oriental languages have problems with the english “L” sound. Their language doesn’t have that sound, and their brain actually loses the ability to process it as they get older.
I seem to recall this loss of sound skills as happening at a much earlier age than the 11 to 14 ages from previous posters, more like around ages 5 to 8. Or, maybe the process starts around ages 5 to 8, and is completed around 11 to 14. After this happens, they have proven that different areas of the brain are activated when you learn a different language, so it’s more difficult to learn a language after this age because your brain does it differently. An oriental adult learning the “L” sound actually learns it differently than an oriental child would, if they were exposed to it.
A baby adopted from another country would be able to learn the language of that country more easily, but only if it spent enough time in that country to learn a lot of the sounds and the base skills required to make the language. I don’t think they would have to know the language itself, just have some roots of it stored in their brain prior to when this switchover of how we learn languages occurs. Just being born there wouldn’t be enough to do it though. The baby needs to spend enough time their to have a few things about the language stored in their brain.
Another good example would be the french “R” for which there is no american equivalent (as my high school french teacher put it, combine the sound of an R with hacking up a furball). A baby born in France would have to be there long enough to learn and recognize that sound before coming to America. Then if it tried to learn French later, it would know this sound already, and wouldn’t have to struggle to learn it.