Certainly you can learn to distinguish languages even if you are raised monolingual, but that is a learned ability. You've still lost the innate ability that yo

Certainly you can learn to distinguish languages even if you are raised monolingual, but that is a learned ability. You've still lost the innate ability that you were born with. Also, there's no reason you can't start your child young with a second language. The French, in particular, are more than happy to train your child in French from a very young age pretty much no matter where you are in the world (search for Alliance Francaise). My wife and I have different native tongues, and we intend to have our child go to a school where the language of instruction is a third different language. We have friends who have raised their son speaking French, Spanish, English, and Turkish. He is now learning Mandarin and is the most articulate 8 year old you've ever met. It really is worth the effort.