Latin was the original language of Justinian's name. He was born Flavius Petrus Sabbatius Justinianus in the Latin-speaking province of Illyricum. His name was later Hellenized to Ιουστινιανός (Ioustinianos) when he became emperor of the Byzantine Empire, which had Greek as its official language.
The Conqueror's Campaign: A Quiz on Justinian, Magister Militum per Orientem