In your expert opinion at what age do you think you should start introducing the second language?
from the beginning.
I am not going to get technical with words- you can google that yourself, however- the part of the brain that assimilates language and speech etc (neocortex) from birth if a child is immersed in two languages this part will allow the languages to grow in the brain together. (I hope this makes sense) I guess you could say the brain files them together.
If a child learns only one language and later on in life learns another (I believe it is after age 13- but I could be wrong about that) the neocortex will categorize the first language and the second language separately. They may know two languages but how the brain categorizes them is different.
It is totally possible to be bilingual at a later age- however, it is easier for the brain to do this from the very beginning when it is still developing.
I am not sure what you mean by "the baby will become confused" All languages are confusing for children as they learn and grow.
My immediate experience is my family.
I have two girls- girls typically learn languages faster than boys- this is not a hard and fast rule but statistically it is the case.
My eldest was born in Canada. From the beginning my husband and in laws only spoke Polish to her. English, however, was the dominant language. When she was 3 we moved to Warsaw and she went to an international school where her teacher was Polish but spoke English. My daughter was picking up these crazy pronunciations and one day we were driving and she asked me what that bright yellow thing in the sky was called- that summer we spent in Canada. She now goes to a different international school where the teachers come from all parts of the world- as do the student.
my youngest was born in Warsaw. Polish was her dominant language until she started international school and spoke English with her classmates.
My eldest always new who to speak Polish to and who to speak English to.
My youngest often mixed up the languages but now she doesn't.
It is much like learning to talk, read or write. Initially there is confusion (not major confusion) but as the brain grows the languages grow and develop too. Ultimately what is the big deal if children make grammatical or language errors when they are young. It is part of the learning process.
How can children differentiate between the two? I have been told that if you introduce the second language to early the baby will become confused.
The children differentiate if each parent exclusively uses their mother tongue while addressing the child. Children can strongly identify their parents voices and when they are being talked to much before they can speak themselves (assymetric language acquisition). Haven't had the opportunity to put it into practice yet... :)
I have put this into practice two times with no problems. Their brains are sponges- they can accommodate a second language with no problems.