Try Japanese locale via launch options
LANG=ja_JP.UTF-8 %command%
Didnt work, Still its in squares any specific fonts to install ?
I also tried changing the whole system locale but that too didnt work.
Installed cjkfonts using protontricks and also changed the locale still doesnt work
See on the top right my username and all other players having japanese names are just presented in square.

Tried that on Fedora and RHEL both
This is still an issue.. please do some upstream proton improvements for CJK support ? Just the font display, maybe not IME.
LANG=ja_JP.UTF-8 %command%x1 2020-01
Japanese fonts not working under any version of proton in Tekken 7. they just show squares instead of japanese fonts. Installed all fonts using protontricks or winetricks also didnt work.