@Geco: Yeah, my view is that there isn't a single ereader that will give an experience that isn't tedious or complicated for using both Japanese and Italian, or English for that matter.
If you can afford it, my recommendation would be to buy a Sony PRS-650 in Japan (20,000 yen) for your wife, and another ereader like Kobo for yourself. Then the Japanese ereader would also be linked in to the ecosystem of the Japanese ebook market, which would probably be beneficial also.
My wife is Japanese too. I have suggested to her that she buy an ereader so that she has access to new Japanese books, as paper Japanese books cannot be bought where we live in New Zealand. But currently she is content just reading the bunko that she borrows from her Japanese friends. That has kept her in stock for a couple of years, but she will have read all the nihongo books in town eventually
About the current ebook market in Japan, my wifes view was that was the selection was a bit limited so far. Sony just relaunched it's ebook business in Japan late last year, so I expect the market to grow, like it has recently in English.