My son, who is at McGill (US) speaks both French and English-he uses them both interchangeably and at his whim. The school is Anglophone-all English. In town, English alone is fine. Though it is also good to know French. You will be greeted in the shops with Bon Jour, Hello. If you answer Bon Jour, they will speak to you in French. If you answer, Hello, they will speak to you in English,
But-if you want to learn and practice French-Montreal is a good place in which to do it.