Open a support ticket with Atlassian.
If standard Chinese characters cannot be displayed, then it's a problem with the Confluence Cloud platform and Atlassian need to update their internationalisation files to support Chinese users.
If you are hoping to use non-standard characters with non-standard fonts, you are likely out of luck, because it is a non-trivial task to add custom fonts to Confluence Cloud. It is however very straightforward on Confluence Server.
You will need to go to the Marketplace and look for a plugin. i am afraid since you are a Cloud user there won't be that many add-ons. Click on the link below:
All the best,