This has happened to me before on my iPod touch, both the app and iTunes stores turned into the Japaneese version. It was really frustrating. I tried everything and the only thing i could get it to work was doing a full restore.
I remember accidentally changing my iPhone 4 language. It was so messed up I had to call someone else with a phone to help me thru the menus to get it back to English! I will NEVER do that again!