The control panel will be displayed in English in any of the following cases: when you have logged in using an IIJ ID account and the set language of the IIJ ID account is English, and when you have logged in as an operations manager and a language other than Japanese is set as prioritized in the web browser’s language setting. Switch the language setting to Japanese, and then log in again.

See here for how to change the language setting for the IIJ ID account.