Can I use both Japanese and English?

You cannot switch all display text and emails between Japanese and English based on each viewer.

The breakdown for each item is as follows.

Fixed storefront text (such as text around buttons):The display switches automatically according to the viewer's browser language.
Campaign setting Note Text: Displayed in the language as entered. If entered in Japanese, it will be displayed in Japanese even to viewers using an English browser.
Notification emails: Sent in a single language specified in the app admin panel's language settings. Emails cannot be automatically adjusted based on the destination country or language.

For instructions on changing language settings, see the help article below.