Languages of the Ottoman Empire
The language of the court and government of the Ottoman Empire was Ottoman Turkish, but many other languages were in contemporary use in parts of the empire. Although the minorities of the Ottoman Empire were free to use their language amongst themselves, if they needed to communicate with the government they had to use Ottoman Turkish.
Lingua franca
The use of lingua francas has existed since antiquity. Latin and Koine Greek were the lingua francas of the Roman Empire and the Hellenistic culture. Akkadian (died out during Classical antiquity) and then Aramaic remained the common languages of a large part of Western Asia from several earlier empires.