Hi!
Translating TPT into Japanese won't be easy, but it's been done for Korean in a Naver TPT community. The font.h should not be edited directly - there is a program that lets you visually change the font and then makes the font.h data for you.

I have rehosted and sort of semi-maintain it here: https://github.com/boxmein/tpt-font-editor .

A tutorial on how to use the font editor (written in English) is in the readme file. You can download binary executables for Windows and Linux here: https://github.com/boxmein/tpt-font-editor/releases/tag/v.1.0.0

The first problem you will encounter is that the TPT font can support up to 255 characters unless modifications are made to the code.

There's a Korean build out in http://cafe.naver.com/powdertoy , which requires registering (and of course understanding Korean!). However, you may have a chance there to find out how they modified the Powder Toy executable to support a larger range of letters. There's been a few attempts like mniip's commits to the code, so there is something to build on.

...It would still be nice to use English in the save community since that means people of all communities can understand each other at around the save level. :D