Unless you're doing a match somewhere where no one can speak a word of English you'll be fine with English. I just reffed 2 predominantly Francophone teams and had no issues using English. I tried to use French when possible, but I just don't know all of the proper terms, and I find it takes me too long to give instructions outside my native tongue on the field. Many of the terms are similar enough that even if a specific player doesn't speak a word of English he'll understand stuff like Advantage, which is just Avantage in French or roll (rouler), no (non), etc. At the end of the day I'd say that probably half the terms get used in English anyways. I've never heard a ref yell "mêlée spontanée" in a rugby game. They always say "ruck". Also, for scrum cadences, anyone who's ever played in the front row knows that there is a 3 word cadence. Who cares what language it's said, they know that after the first word they crouch, after the second they bind and then after the third they're set. You could literally call the scrum cadence in gibberish and the pack would know what to do.
