My DTD has just started saying 'oui', its her 5th word (maman, papa, nanna, and no-no) and she's been saying the other 4 for about 3 months. she is now 17m. DTS has no words at all! I think DS1 had a couple of French words as well as his English, in fact he seemed very equal until he was 2, the French fell behind a bit once he started using sentences.
not sure about babble sounds though. DTD is the only babbler and it sounds the same whether to me or her father.
Ooops - I never noticed until they used actual words from their other language, which was probably around 14 months with the boys, who were born here, and a few weeks after we moved her with DD, who was 19 months and already speaking well in English when we moved
DC3 stopped using any German for a while at about 22 months, while his English surged ahead, for no logical reason (no trip to the UK in that time or anything) and then after a few months of seeming monolingual his German started to come on again, both understanding and speaking. He's 2.5 now and his English is still ahead but he does fine in German (minority language - English - at home, just started playgroup in German twice a week, no other child care but very immersed as we don't spend time with expats. Older kids are fully bilingual without any problems, at school and Kindergarten in German.
No idea on sounds before proper words though, very remiss of me! DD did revert to babble a bit when we moved, which annoyed me as I was very proud of her speaking in sentences so early, but I learned it is normal when acquiring another language as a toddler.
what luvmykdis said. And it's the same in Swedish: the most distinctive sounds are usually the last: think tj, skj and the rolling r: my monolingual nephews didn't really get those until they were school age
dc sounded like monolingual native children from the start, but like monolingual native babies iyswim
sometimes I did take it as interference with English- until I heard my nephews
it depends on whether you're good at hearing sounds or not. By 18 months perhaps? Go through the phonemes of each language. The most distinctive ones in English are also the last to be perfected usually, the vowel sounds you should hear more differences earlier.