In the past I have been on tours of Thailand and the guides would in many cases tell you they don't know English very well. It wasn't a problem for me as I had decent understanding of the Thai language. However, anyone coming just for a holiday and getting a guide that speaks very little of the tourists language would certainly decrease the pleasure of the tour.
Thailand needs tour guides to have a good command of the language that the tourists they are with use. Not a lot of good for a guide to speak Malay if the group is Japanese for exxample.