No need to write a program for that, some scripting or manual typework is sufficient.

Download the latest planet dump.
Update with latest data by applying change sets using Osmosis
Extract Thailand from the planet using Osmosis and its bbox functions
Pull the updated planet through Splitter to create tiles
Compile the tiles into a Garmin map using Mkgmap.

See OSM wiki for detailed usage of Osmosis, Splitter and Mkgmap.