Googled it, here a guide from the first site:
Since there were a lot of question lately, about how to play the >Japanese version of the game, here is a short tutorial how to >obtain it. First things first: Yes, you can have both versions (JAP and >GLOBAL) on the same device and it is possible to play both >versions on any device. The key to obtaining a fully functional Japanese copy of the game >without any further trouble is a Japanese iTunes account. Yes, >there are other ways, e.g. downloading the latest version from >specific websites and updating it with QuooApp etc., etc. but the >safest way to enjoy the game without any complications when >updating it, data corruption or whatever is to get a Japanese >iTunes account. 1. Open iTunes on your computer and log out your current >account if you have not already done so. 2. Go to the App Store and scroll down to the very bottom of the >page. 3. You’ll find an option to change your region on the bottom right. >Change the region of the App Store to Japan. 4. Look for a free App in the Japanese App Store and click the >download button. (This step is important to skip choosing a >payment method) 5. iTunes will ask you to enter your log in data or to create a new >account. You want to create a new account. 6. In order to create a new account you will need a “fresh” e-mail >address that has not already been used for another iTunes >accounts yet. 7. Fill out the blank. Be sure to select “None” (the rightmost >option) as your payment method in order to skip entering >credit/debit card information. !!!You can use fake data for the blanks, the only thing that has to >be kind of valid is the address and phone number. Just use >Google to find valid data!!! EXAMPLE: Zip Code: 100-0001; City: 東京都 (Tōkyō-to); Prefecture: >Chiyoda-ku; street: The Peninsula Tokyo 1-8-1; phone number: >080 1234567 You'll get a confirmation e-mail. After you've clicked the link you >can download the Japanese version of One Piece Treasure Cruise. >You can log back in with your original account. The only occasion >you will have to log in with your Japanese account again is to >update the game.