NextCloud Office cannot display Chinese and Japanese correctly

Frankly speaking I have never tried. But I think it is possible to install additional fonts, say, from Microsoft, which are more universal or find existing universal font and enable it as default.
I think you should check this thread: Installing new fonts in Collabora - #13 by chiuyau
and https://help.collaboraoffice.com/latest/en-US/text/shared/optionen/01010700.html

I am sorry, I have no time at the moment to try this proposal by myself now but maybe will do it later.