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.
Croat Muslims
In 1585, a traveler and writer Marco A. Pigaffetta, in his "Itinerario" published in London, states: "In Constantinople it is customary to speak Croatian, a language which is understood by almost all official Turks, especially military men." Crucially though, the lingua franca at the time among Slavic elites in the Ottoman Empire was still Old Church Slavonic. For Italians traveling through to Istanbul, the language of the Slavic Croats was often the only exposure they had to any of the Slavic languages; indeed, Bulgarian and Macedonian dialects were far more common in Istanbul than Croatian.