First of all, it is helpful if you understand French because all maps, menus, and names are of course in French.  But being fluent is not a necessity because most people do speak at least a little English.  In fact, I have spoken French to people and then they have responded in English!  This is because they just figure it will be easier.  Honestly, I would always start and try to speak a little French if you can out of respect and that will usually get them to treat you kindly.  In general, the French do not like it when people do not at least make some attempt or effort to speak French.  So be fair, say whatever you can, but you probably will not have to say much.  There are still some people who will not speak English right away, such as owners of small shops, or people working at markets, so it is a good idea to know some of the basics before you leave.

In a related issue, one of the biggest rumors is that all French people, especially Parisiens are snobby and rude.  Well, I can only speak from personal experience, but honestly I have never been treated rudely by anybody in Paris.  In fact, there have been more instances when I have been treated almost sickeningly kindly.  I have great affection for the people as well as the city because of this.  The fact is, the French will respect you if you make an effort and are honest.  If you need someone to speak English, then ask, but please, do not just start speaking English assuming they will follow.