@Shinora: I had the same problem you have several years ago. Then only a very small part of the site was in English. Luckily, now much more info is given in English, but not everything as you have noticed.
What I did to solve my problem was send a message, in English, to Amazon via their webform (found it by comparing the amazon.com site with the amazon.co.jp site, they use(d) the same software). Since then, Amazon.jp sends me e-mails in English and the problem with the validation of my credit card was solved by sending a fax with the details.
You can send a mesage to Amazon.co.jp via
this page. You have to choose a subject (Payment) and enter the order number. Describe the problem in the comment section, but don't submit any credit card numbers. That form is not a secure submission method!
Amazon should contact you with instructions in English.
Hope this helps.