If you want the Thai Keyboard then buy a Thai or Malaysian unit

It is possible to load a Thai keyboard on USA or European units but only of certain firmware and not the new ones
You can get a Thai keyboard then when you switch on the Thai language you get a bricked Nuvi
The other way is to load Mal/Sig firmware to unit first this also is fraught with possible bricking
You need to do a lot of reading and looking around before you go any further

This is good free advice, please be careful.