Great topic Fabienne. This is something my wife and I discuss on a regular basis. We are both francophone from Montreal but now live in Toronto. We decided before our first child was born to speak to him, and amongst ourselves at home, strictly in French. I know far too many French speakers whose children, though they can understand it, can barely speak it and I feel it's a shame given how easily certain kids can learn multiple languages. Now that he's four, Noam is fluently bilingual but probably has a more extensive vocabulary in French. He speaks English with his nanny and playground friends...
English is a relatively easy language to pick up and one that will be learned simply by virtue of living in North America. Other languages OTOH cannot be learned without a sustained, conscious effort when living in a predominantly anglophone environment. It's a struggle and I know many families where only one parent speaks another language and after a while the children simply stick with English all the time. One of the things we have done in addition to the French at home policy is to show all DVDs in French (in Canada, they all come with both language options) and sending him to French school (he's starting preschool tomorrow
Before the age of 5-7, children have an amazing ability to learn different languages that then deteriorates over time according to some theories. That said, some kids (just like adults) are better at learning languages than others. Noam is remarkably adept at articulating very different sounds and words. I read books in Hebrew to him and he's learned quite a few words. He also has Spanish books and videos and has learned a few words there too. I hope he will speak at least three, if not four, languages by the time he reaches adulthood. My daughter Annaleah (20 months now) is not quite as verbal as her brother.
My advice is to be persistent and consistent in only speaking to the child in French or other. When he was smaller, Noam would translate into French what his nanny would say to us in English because in his world, we speak French and his nanny English - it was very cute. Now he understands and sees me speaking English to people outside but will still speak to me in French. I hope it continues... we're trying our best.