Well, you should have no problem speaking English in Crete, most people speak it there because of the tourists.
Tell me when are you going and I'll tell you if there is an OFI game at the time.
No, I don't think it gets sold out like... ever. They play in the enormous Pankritio stadium.
(By the way, in any given weekened, if OFI doesn't have a homegame, Ergotelis does. Ergotelis is the second team of Iraklio, leaders of the second division and they are meant to play really nice football... and their fans create a very nice atmosphere. So if you can't catch an OFI game, you can catch an Ergotelis one).