I think the Japanese version of the online languagetool doesn't meet passable standards to be live.
I get "no errors found" for anything I type including this:
I also tried real Japanese words with completely scrambled grammar, the wrong particles etc.
I think it would be better to disable Japanese from the languagetool or have a very heavy specific disclaimer about what does and doesn't work.
Sorry, I didn't mean this post to sound gruff, and I hope it doesn't, but I think there's a point where you have to say the tool does more harm than good.