we do provide fcitx with bazzite by default so you should not have to add that i believe. dont think it will work using the steam keyboard still though as that is something valve will have to fix, im not familiar with typing out CJK characters, but i know ibus or the newer fcitx is used for it
you can try utilize fcitx which is included you can launch the config with not sure about how it is in gnome apparently chinese should already be there
and i ask, did you try enable fcitx virtual keyboard in input settings on the kde desktop and configure it using and see if that lets you type in traditional chinese?