This blog post
suggests that Japanese text is possible to some extent, if you install an appropriate font.
But Japanese certainly is not a language that Kobo support, and I doubt you would get the kind of polished Japanese book reading experience that you would get with an ereader for the Japanese market, like the Sony PRS range
Japanese novels are usually printed in the text top-to-bottom, right-to-left format. I am not sure the Kobo can support that.
However, dialectal marks for Italian shouldn't be a problem.