Steam requires several IBus engines to make the input methods (IM) work:

• ibus-table-cangjie-lite: It exists in nixpkgs as "ibus-engine.table-chinese" but the version is older than the one used by Valve (which includes a patch to limit unused tables). This package is for the Traditional Chinese Cangjie input method.
• ibus-pinyin: This package does not exist in nixpkgs. It is for the Simplified Chinese Pinyin input method.
• pyzy: This package does not exist in nixpkgs. It is a dependency for ibus-pinyin, used for Pinyin and Bopomofo conversion.
• ibus-anthy: This package is for the Japanese input method, with a patch from Valve. The upstream package exists in nixpkgs.
• ibus-hangul: This package is for the Korean input method. It seems Valve uses the upstream package directly. We can install it from nixpkgs.

I have tested that the Steam IM works after installing ibus-table-cangjie-lite, ibus-pinyin, pyzy(Works in both Gamescope Session and X11 Desktop Session):