Hello! Thank you very much for your kind words! ^^
Yes, you can use Card Duel System to create a game in Japanese.
The UI text used by the system is customizable, so you can translate the interface into Japanese and adapt the different labels and messages to your project. The card content can also be created in Japanese.
Just make sure that the font used by your RPG Maker MZ project supports Japanese characters so they are displayed correctly.
So yes, creating a fully Japanese Card Duel System project is absolutely possible. :)
If you have any questions while setting it up, feel free to ask!
Best regards!
Undermax Team