The 'why' will come when he's sober.

With a grin, Oikawa pulls Kageyama towards the stairs. It's not that late, so there are people walking around, watching TV or chatting amongst themselves in Spanish, a few wave at him, and he waves back. One person asks what he's doing with Kageyama, and he responds with, "Sólo voy a ponerme al día con mi kouhai."

'Kouhai' just translate well into other languages because Japanese is one of the most respectful languages in the world.