Most probably the fonts in the firmware needs to be modified also the part dispaying characters from the map. (I am not sure how the gpsmap 62 firmware is structured or the files inside the memory). I have done simmilar thing in older unit firmwares - vista hcx and gpsmap 60csx when adding cyrillic alphabet.
I even see that neadrenalin also have made Thai firmware for gpsmap 60cs. That is cool!
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On that page you can find useful information despite that is quite old.