I live in Montreal and you are right that you would need to understand some French but only to read signalization, some store names and public advertisement. Signalization of course is important if you are driving but an app like this won't help you (you are holding the wheel I hope). Besides that, almost any store or institution has bilingual staff (and when they don't most French speakers will understand basic English). Don't let that hold you back... unless you hate snow and cold winters ;)