Thanks for posting in the Spotify Community!
If you want to listen to the Japanese Spotify you need to change your country to Japan. Follow these steps to change your country: https://support.spotify.com/is/account_payment_help/account_settings/how-can-i-change-my-country-setting/
There's two things to keep in mind though:
Other than that there is no way to switch to Japanese I'm afraid. Tell me if this works for you!