The reason behind is the post I've just renoted/reposted (see end of this very post). If you see the image sent by the ap12's account, the Mastodon interface is in English. That indicates that the person controlling the account understands English. However, what are the chances that a Japanese kid under the age of 14 understands English? Very unlikely.
Thus I conclude there is a very high chance the entire spamming was done by OP, who impersonated ap12.
@hakui@tuusin.misono-ya.info There's also the ability of impersonation, aka. oh my god I forgot I needed to pretend to be an underage kid now I need to send a few stupid posts before the admins ban me
@madomado multiple behavioral clues and word choice in his posts, and you take a screenshot of what could be a sandboxed session defaulting to the remote instance's default language as the conclusive proof?