A lot of people do speak English now, but if you know any French, it will get you much further. At the very least, it'll make it easier to chat rather than simply transact (I found Marrakech taxi drivers - and Fes touts, but that's a different story - to be an incredibly friendly, chatty and fascinating bunch once you get past the negotiation stage).

(Arabic, naturally, will get you further still, but I'm assuming you wouldn't be asking the question if you were fluent in it!)