They speak English and are maybe a little gratified when they do so successfully. I doubt it makes any difference, but your Thai should be quite polite and up to the task. Really, the papers are the issue.
I give a quick, polite Sawadee Krub, smile, and a respectful little wai. I figure, can't hurt and may be a start on the right foot anyway. Personally, I sympathize with what they must put up with daily. I can say that with me, at least, the IOs are always polite and professional. My papers are always in perfect order, however, neatly typed. I'm normally a messy person but not in this case.