Montreal is a very international city, so you are sure to find people from various origins here. However there are some general etiquettes to consider when interacting with the general populace.
French is the official, and the most commonly used, language in Montreal. If you do not understand and cannot speak it perfectly, it would be hard to survive in the outdoors - markets, for example.
Although it depends on the context (a Francophone or Anglophone community), it is more appropriate to start off a conversation with a stranger in French. If you think your French skills are poor, it is polite anyway to inquire "parlez vous anglais" (do you speak English?) before starting off in English.
Canadians, especially Montreal residents, are quite tolerant of people from different linguistic and ethnic backgrounds. Some are even able to communicate in languages other than English and French. Whichever language you speak, you are sure to be welcomed and accepted in Montreal - as long as you respect the other person's language priority and skills.
It is also advisable to get a little know-how of a specialized neighborhood you intend to visit, eg Chinatown or Little Italy. Having background knowledge of the traditions of these nationals would be helpful in initiating and carrying out conversations.
Canadians are polite people. Asking for directions, demanding services, and financial transactions, all are warmly responded to. It is not uncommon to hear "thank you", "please", and "sorry" in public places.
Tips are generally not included in the bill. A tip of about 15% is customary on services such as hotel staff, bars, taxis, and hairdressers.
Montrealers pay attention to attire. When you are out on the street, inside shopping complexes, and, especially, in restaurants, be sure to dress appropriately.