After traveling the world a bunch, I've learned that you can travel almost anywhere in the world and not know the language at all, and still get along fine. If you make a genuine effort to be a kind respectful guest/traveler, people all over the world will go out of their way to help you. All the core needs can be expressed universally by humans. A little money and small gifts of course helps to repay kindnesses.

I was traveling with a guide who knew Greek and had visited Mt. Athos at least tens of times. It was very valuable as he knew monks from different monasteries and arranged visas and accommodations. But during my trip I also met few travellers who had arranged everything themselves and didn't speak any Greek. You can stay at a monastery one night without a charge, meals included but I think it's suggested that you book it in advance because some monasteries have limited capacity for pilgrims.

You can get by with English, but you won't understand the service (which is in the monastery's main language, e.g. Greek/Russian/etc).