Here's some additional information:
In archlinux on the same machine, I used to use flatpak steam and configured the CJK fonts well. However, after a update of steam (maybe two weeks ago), the same problem appeared: CJK fonts of the client worked fine while in unity games CJK fonts were missing.
Hello @zeauw, this issue is already being tracked at https://github.com/ValveSoftware/steam-runtime/issues/555. Closing as a duplicate.
Your system information
Please describe your issue in as much detail as possible:
CJK fonts work fine in steam client. However, in unity games without fonts embedded (like rimworld and streets of rogue), the CJK fonts are missing.
In client, CJK fonts work well:

In unity games, CJK fonts are missing:
