Firstly try to use Garmin webupdater.
Follow easy menu selection and check Thai text and Thai voice.
check box for "all asian text" or something like that
if you have thai characters but not the keyboard, try this:
Add these two lines into \Garmin\Text\Thai.gtt in <hdr> section:
And change keyboard style in system setting from QWERTY to ABCDE
If your device doesnt has this possibility and cant display thai characters
you should flash it-
Flashing refers to changing of a firmware of a non-localised unit to a localised one. A word of caution if you want to flash a non-localised unit to a localised one. There are certain risk involved when flashing a unit. Doing it wrongly can result in your device becoming a useless paperweight. Besides, it will also void your warranty. Sometimes even when an unit has been successfully flashed, its performance can be erratic and causing errors during navigation-