After a few months, I decided to dig a little more into the UE4’s code.
Basically just adding a few fonts file into LegacySlateFontInfoCache.cpp and the editor will display some Thai text. It’s not perfect yet, but I think it looks good enough.
The font I use for this screenshot is Sarabunfrom Cadsondemak. It’s a variant of TH-Sarabun PSK, released under OFL terms.
I am actually interested to send in a PR for this change, however since font files are not parts of the source file hosted on GitHub, I can’t just send one and hope that it will work correctly. So can someone suggest how and what should I do to get this one in?