Check out this:
Additionally, in most widgets as well as on the document template itself, a style can be defined. This will be discussed in the topic Style If you’d like to show text in a language with uncommon characters such as Arabic or Thai, make sure you select a font in the style editor that supports these characters. ‘Tahoma’ is such a font.
Check if you have a font that supports the japanse charaters.
Also the document template functionality is limitted there is an idea on the forum to improve this, might be worthwhile to vote: https://forum.mendix.com/link/ideas/1097