In their first year, well before they say their first real words, babies babble many different sounds as they experiment with their voices. Supposedly, babies actually produce and experiment with ALL of the sounds, including those not in their native languages. As they mature, immersed in their native language, they start limiting themselves to only the sounds that they hear and eventually lose the ability to even hear many sounds in other languages. When I was visiting in Korea, I would try to imitate a word, to be told repeatedly, “No!” And they would say the word again. I thought was repeating the word I was hearing! Apparently not! You are probably aware that Asians often confuse the sounds “l” and “r.” That sound is the same in their languages and they actually cannot hear the difference.
This is why, if you want your child to speak fluently and without an accent in a second language, he must be exposed to the second language very young in life.