Ardenet responded as follows:
This is likely a Proton version issue. I’m not entirely sure of the specifics, but it’s probably due to incompatibility between the font lookup mechanism in older Proton environments and NIX management. You can try switching the Proton version to the latest experimental version in the game’s management page. That’s how I solved the problem of Chinese character display in my game.
这个应该是proton版本的问题，具体情况我也不太清楚，但应该是旧proton环境中查找字体的机制和nix管理不兼容。可以在游戏的管理页面切换使用的proton到最新的experimental版试下，我有个游戏内中文显示就是这么解决的。