~ is typed by pressing Alt-Ctrl which brings up additional symbols to enter.

However, I don't understand why you are not installing the thai fonts from Extras-testing

http://maemo.org/packages/view/thai-ttf/

You will need to enable extras-testing https://wiki.maemo.org/Extras and reboot your phone afterwards.

The software hosted in extras-testing might not be ready for end users! Please don't play with it unless you really know what you are doing.