The Kodi team does not use github for feature requests at the moment (actually we switched to github for issues about 5 minutes ago...), so your submission was closed immediately.
That being said: for someone with a bit of coding and a bit of knowledge of how Japanese works:
What needs to be done is most likely something similar to what was submitted in this PR for Korean: (see Add Korean keyboard by sportica · Pull Request #7952 · xbmc/xbmc · GitHub )
ór similar to what was done for Chinese, where the input is using a cloud service to transform ascii input to the correct characters. (see [keyboard] support coding input method for chinese by taxigps · Pull Request #5326 · xbmc/xbmc · GitHub )