Recently, I'm trying to fix d-i Han-Unification issue for Japanese. This issue was not fixed for a long time since Debian 9 (stretch).
#1037256 - debian-installer: GUI font for Japanese was incorrectly rendered - Debian Bug report logs
To know about how Han-Unification is harmful for Japanese in some cases, See "Your Code Displays Japanese Wrong".
When building d-i (GUI Installer), you need to build build_netboot-gtk target.
But note that you need recent master because it has nitpick issue with GNU Make 4.4.x.
After that, need to prepare required packages. See README in details.
apt-get update apt-get install -y myrepos git libgtk2.0-dev fakeroot apt-get build-dep -y debian-installer
It seems that Bug#1037256 will be fixed with supporting compressed font. I don't know how to do it furthermore , but I'm sure that Mr. Cyril Brulebois will handle this issue better. :-)
(I thought that creating fake fontconfig cache when building image, then decompress compressed font dynamically might work as just an idea, but it didn't work.)
If you would like to tackle fixing d-i issues as a newbie, it might be better to execute "make reallyclean" before rebuilding image not to fall-in pitfalls.
