I have a kindle 3, had it for about 2 years now and I have to say I love it.
It does not support native Japanese files and there is no Japanese dictionary for this version (and I don't think they are working on one either)
As for PDF of Japanese books, they display like any other pdf, no problems. Sometimes though the size of the kanji on the pdf may be a little small so I have to zoom in and keep it zoomed in if I want it to be legible.
I downloaded several different dictionaries for a few languages including german, french, italian, latin, africans and some other stuff that was available for free from this website, they work with it but the problem is that you can only have one dictionary as the main dictionary at one time so reading books (most of which may have passages in different languages) I have to go all the way to the home screen, into the options and change the primary dictionary so the scroll definition function will work. It is kind of a pain and it does not work with abbreviated words or slang, but for all tiny inconveniences it does give you the basic meaning of the passage.
tl:dr
As I said it does not support native Japanese books and there is no dictionary support on this version, but for most other languages it works great
It does not support native Japanese files and there is no Japanese dictionary for this version (and I don't think they are working on one either)
As for PDF of Japanese books, they display like any other pdf, no problems. Sometimes though the size of the kanji on the pdf may be a little small so I have to zoom in and keep it zoomed in if I want it to be legible.
I downloaded several different dictionaries for a few languages including german, french, italian, latin, africans and some other stuff that was available for free from this website, they work with it but the problem is that you can only have one dictionary as the main dictionary at one time so reading books (most of which may have passages in different languages) I have to go all the way to the home screen, into the options and change the primary dictionary so the scroll definition function will work. It is kind of a pain and it does not work with abbreviated words or slang, but for all tiny inconveniences it does give you the basic meaning of the passage.
tl:dr
As I said it does not support native Japanese books and there is no dictionary support on this version, but for most other languages it works great