Kindle Voyage, released October 2014.
Nowadays, you might want to consider a Kobo. It is on pair with e-ink technology, reads epub directly and much more hackable, see here for custom firmware: preining.info mobileread.com also Japanese dictionary exists
Reading own Japanese content in traditional vertical style (縦書き), is possible. Calibre can not generate vertical writing documents, but files generated with other tools can be managed/uploaded to the Voyage. With PDF files it is possible to read Japanese vertically, but marking Kanji and looking them up in the dictionary does not work. The following procedure produces .mobi files, appropriate for reading and marking.
As input, UTF8 textfiles can be used. Or the .zip files obtained from aozora/青空, a library with free Japanese content, kind of Japanese “project gutenberg”. The following method provides these benefits:
Getting UTF8 txt files displayed on the paperwhite and beeing able to use the japanese→japanese dictionary:
cat scifi.txt | kakasi -JH -i utf8 -f | \ sed -e 's,\[,《,g' -e 's,\],》,g' >scifi_furigana.txt