I'ver never worked with the Thai language, but if your going to do the game in DirectX, OpenGL, SDL, GDI, etc... Then you should be able to create a custom font that allows for all of the characters that you need. Then write up a simple display engine that takes care of the positioning (something like a stripped down version of Uni-Code should do).

This is what I've done for a few locialized web projects, and in the end it worked fairly well. Only downfall on the web was no copy-paste . In a game that isn't such a problem though luckily .