Your native language was not Japanese even though you spoke it fluently, no one else really assumed anything so they never guessed. One evening enough, in the Armed Detective Agency, you were taking a personal call on your break, speaking in a language just about no one there understood.

Osamu Dazai was nearby and the new language peaked his interest immediately, he stood in the doorway despite being entirely unable to decipher what it was you were actually saying. It was quite the ineffective way to eavesdrop.

But he was determined to figure it out and then ask you about it right when you finished your phone call, which luckily was about to happen.