Once someone creates a Thai keymap we can add it to the image. I’m not really familiar with creating keymaps. Maybe the Advanced keymap manipulation section of the user guide can get you going?
If Thai needs a more complicated means to enter text, like an input method, thing are, er…m more complicated.
WRT a Thai font, check out the Noto_regional package in HaikuDepot. It should support also Thai. Alternatively, you can install any TTF by downloading and moving it into /system/non-packaged/data/fonts/ttf/.
Of course, it’d be nicer if Haiku would offer to download the needed font at the “FirstBootPrompt” where you select language and keymap, or at the “Installer” stage. But we’re not there yet…