Correction: No, not essential but someone will eventually treat you like shit because you don't speak french, and it's going to be offensive and feel like overt racism, but it'll happen rarely. </rant>
"Excuse-moi, parlez-vous anglais?" ("Excuse me, do you speak English?")
(phonetically that's roughly "ex-cues-eh-mwah, par-lay-voo an-glays" with a little emphasis on the "glays", just say it fast and it'll sound recognizable)
You might get no. Then you say, "Oh... no parlez-vous francais... do you know where xyz thing is... xyz thing... xyz thing... y'know xyz thing..." Eventually they'll get it. The two-second politeness of addressing someone in French has always been enough for me.