[Steins;Gate] String encoding failed on sc3tools

[ModzabazeR]

I was testing some translation and try to replace text back into its script (.txt --> .scx)
The game is Steins;Gate VN, and here's the error:

Error: String encoding failed. Unable to encode character 'น': not present in the game's character set.
at line 0.

I know this happens because the Thai alphabet is not in the character set, but how or where do I fix it?