BookWalker's registration page is available in both Japanese and English. Look for the language toggle in the top right corner.
You could also login with other accounts like Facebook, Twitter, or Google.
Credit card options include Visa, Mastercard, American Express, and the regular Japanese cards.
Once you are registered with BookWalker.jp, your account also works with the BookWalker Global Store. This is very helpful if you want to look at both the Japanese and English light novels, for example. BookWalker Global Store also lets you pay with PayPal.