Document Templates probably do not work because you are missing fonts for Japanese.
A simpler solution might be to use another document generation tool like https://appstore.home.mendix.com/link/app/114043/
Hope this helps
Add a custom font which has support for Japanese to your project following https://docs.mendix.com/refguide/style#custom-fonts.
In the ‘Custom styles’ tab of the cells which need it define font-family: ‘your font’ ;
You must add one of these fonts, I dont remember exactly which one yahei probably, that supports Japanese
Then follow https://docs.mendix.com/refguide/style#custom-fonts as Sjors already mentioned.
Then in the document properties, you can include the font like. This will make sure the font is available for entire document.