I translated all message into Japanese and I publish it here now.
If this plugin use my Japanese translation by default, I will be happy :)

File is renamed from messages.yml to messages.txt to upload.
messages.txt

Since this plugin has too many translation, I could not test all message.
So some mistranslation may be found.

Wow, thank you so much!

Thank you again, we now have a builtin JP language option.