The details around the Japanese language are murky, to but things lightly. There is debate as to whether Japanese is an Isolate, is a part of the Japonic language family, or somewhere in between. What we do know is that Japanese is spoken by more than 127 million people world-wide and that the language has been around (spoken and written) for quite some time, with the first known Japanese writing dating to around the 8th century CE.
To hear the 1st paragraph of the 1st book, Click Here.
There are many editions of the Japanese Harry Potter books. Below are two, however, I have many others and will be adding them as my time permits.
Like many other translations by this point, there are House Editions available - they are lovely.! I love the green color they used for Slytherin. While I don't own the other 3, I am sure they're produced just as nicely.
ハリー・ポッターと賢者の石
ISBN: 9784863894631
ISBN: 9784863895201
ISBN: 9784863895218
ISBN: 9784863895225