We just wasted an hour going to and from Angkor Wat on our first day in Cambodia. The guide our hotel arranged for us did not speak any English whatsoever and we asked to turn around and go back to the hotel. Now the hotel says they'll "try their best" to find an English speaking guide but its difficult in the busy season. I had heard from multiple sources including the hotel that finding a guide on arrival is not a problem.
Just a warning to others. I wish I had booked a guide recommended here in advance.