Hi @sasha2 — I guess we don't have too many users in Cuba yet, but I can speak from my own experience traveling in Cuba.
You by no means need to be fluent in Spanish to enjoy a trip to Cuba, at least in the somewhat more touristic areas. Some basic Spanish can come in handy for getting directions and negotiating prices with "colectivo" drivers for traveling between cities.
That being said, the more Spanish you can speak, the more meaningful interactions you'll be able to have with the locals... 