Montreal is a French city -- the largest French-speaking city outside France. Bracketing your English with an opening Bonjour and a closing Merci is not only good practice but a courteous gesture. Don't worry about your accent; plenty of Montrealers are not pure laine native speakers themselves.

Almost all Montrealers speak excellent English, which can be a problem if you're trying to learn French. It's hard to get two sentences out before they switch to English on you. However, if you can clue them in that "Je suis un Américain", they'll be quite impressed that you're trying and will help you out. Otherwise, they think you're an Anglophone Canadian who never bothered to learn French, and they have no patience for that.