sorry I thought your keyboard configuration files were like in other previous devices
30vm23o

usually there is a text folder that stores default and custom messages and keyboard layouts. These are text files with .GTT extensions. You can edit them with a text editor.
I donnow about glx files and if you can edit them-
I am not quite sure what you mean by “have thai characters but no thai keyboard” but my unit can display thai map, pois with thai characters. no problem with that.
Some devices have thai characters but not the thai keyboard (US sold devices) that GTT config works with those devices