This evening Pottermore announced that the Harry Potter ebooks are now available to purchase in the Japanese language from the Pottermore Shop. They also made a brief comment that the Pottermore Experience (Pottermore.com) would follow suit later this year with the release of the site in Japanese.
From the Insider post:
The Pottermore Shop is growing; we are delighted that even more people can now enjoy the Harry PottereBooks as they become available today, for the first time, in the Japanese language.
The Japanese editions of the Harry Potter eBooks are published in EPUB3 format and can be read on any EPUB3 device or reading service, with Japanese font enabled.
Pottermore.com, the free-to-use experience centered around the Harry Potter stories, will also follow suit later this year, offering Japanese Harry Potter fans the chance to explore and discover new writing by J.K. Rowling and immerse themselves further in to the magical world of Harry Potter.
You can visit the Japanese Pottermore Shop right here.