The name İstanbul was first mentioned in the 10th century in Armenian, Arabic and then Turkish sources. The name comes from the Greek term "εἰς τὴν Πόλιν" or "στην Πόλη", which means " in the City".
This name was used in spoken language before the Ottoman conquest of the city, and also, used during the Ottoman period, although the official Ottoman name was قسطنطينيه, (Kostantiniyye).
So, technically both are correct since both Greeks and Turks actually used...both.It was not until the creation of the Turkish Republic in 1923, Istambul was declared the sole official name of the city.
If this is still being worked on, I would suggest this as the western Balkan border between Hungary and the Ottomans;
Edit; forgot to add spoilers to the image.