Japanese system-wide spellchecker
If you have .dic and .aff files, just put them Library/Spelling or Home/Library/Spelling, making the folder if necessary.  Apple's instructions are in the setup area of system prefs/keyboard/text/spelling.

I have not seen such files for Japanese, but they may exist somewhere.  MS Word has Japanese Proofing Tools usable with that app.

Some info on how to make spellcheck dictionaries is here.



If you need reference and translation dictionaries, Apple provides those in the Dictionary App: