"Zangi" is actually a Persian derogatory word used in a negative way to describe black people. It refers to the black slaves from Zanzibar and East Africa. This expression has its origin in Rumi's Mathnavi (book 4, section 53*). A verbatim translation of Rumi's verse is "It is better to put a sword in the hand of an intoxicated negro than "that knowledge" should fall to a worthless fool.“
I can just imagine how the East African delegations and in particular, the members of the Zanzibar delegation in the UN would have reacted, had they understood fully well what Rouhani was referring to! The translator definitely saved the day and avoided a huge diplomatic egg on the face for Rouhani.