if you learned to speak thai in isaan, as i did, you will often have people do this, since they don't expect a farang tio be speaking lao! (i got laughed at in a bank in bangkok for speaking "k'men"!)
whatever happens, don't let this put you off. concentrate on getting your vocabulary up and learn the rest by conversing with the retards as much as possible: you will soon get the hang of it, and once you do, you will find there is a point where you will "click" and from that point on it becomes fairly easy.
good luck!i