Lingua franca
During the time of the Roman Empire, the lingua francas were Greek and Latin. During the Middle Ages, the lingua franca was Greek in the parts of Europe, Middle East and Northern Africa where the Byzantine Empire had much influence, and Latin was used in the rest of Europe.  Latin was used as the common language of the Roman Catholic Church.  During the Second Vatican Council, Catholic liturgy changed to local languages, although Latin remains the official language of the Vatican. Latin was used as the language of scholars in Europe until the early 19th century in most subjects.