BaZaRa,
It seems to me that the font you are trying to use does not contain the appropriate characters. There may only be three issues (I may always be wrong): wrong input, wrong font, application that does not support your encoding.
Since AC does support your encoding, it must be the other two. Please post your system/AC version data. Did you install all patches/bug fixes for your AC? (I don't have time to go through the fix list so can't say if there were any font problems).
I attached the screenshot with both my native Polish characters and Thai characters. Interestingly, I don't use Polish with AC, but I noticed that some Polish capital characters could only be used with capslock.
Good luck
EDIT: there's one last chance that if you have a Thai copy of Mac OS, your Thai distributor messed up bad with the "patch" that is responsible for system language. Some people also install some third-party "helper" applications that make their input totally incompatible with everybody else's. If you have a chance to install AC on an English/US Mac, do it and see what happens.