Istanbul is not Constantinople.
Both are names for the Queen of the Bosphorus, but Istanbul is not Turkish for Constantinople. The historical Turkish translation for Constantinople is actually Kostantiniyye.
Constantinople means The City of Constantine, after the city’s founder (and shameless self promoter), the Roman Emperor Constantine.
Istanbul is a Turkish colloquialism derived from the Greek in the city or to the city – from the Greek practice of referring to Constantinople simply as The City (Polis).
Istanbul did not become Constantinople’s official name until the founding of the Turkish Republic 1923, when it displaced Kostantiniyye. To encourage the adoption of the city’s new name (there was some hesitation, especially in Europe), the Turkish Post & Telegraph service stopped delivering all mail that did not use it. Needless to say, the draconian measure succeeded.
Modern historical texts commonly label Constantinople the capital of The Byzantine Empire. Technically, Constantinople was never the capital of the Byzantine Empire at all, since the term Byzantine Empire was introduced only in the 19th Century, some 400 years after the Constantinople had fallen to the Ottomans. Byzantium called itself The Roman Empire, for that is what it was: The Eastern Roman Empire that survived nearly a 1000 years after the Empire of the West and its mother city had faded. Western contemporaries thought of Byzantium as The Greek Empire, a reference to its predominant Greek culture, population and language. To the people of Asia Minor and the lands to the East, Byzantium was simply Rum (pr. Room) – Rome. Its people were Romans. And they continued to be called Romans even after Constantinople became an Ottoman city. To Asians, the Ottoman Sultan was better known as the Sultan of Rome.
The Western world referred to the polyglot, multi-ethnic population of the Ottoman Empire as Turks. Turk means strong in Old Turkish, but until the 20’th Century, the West used it as a pejorative! The Turks preferred to call themselves Ottomans, or correctly, Osmanlis, after Osman I, who transformed his tribe (of Turkish origin) into a kingdom that eventually became a superpower. It was only in 1923 that the Empire of Osman breathed its last and became The Republic of Türkiye: The Land of the Strong. Turkey is an English distortion of the Latin Turchia. Any similarity to the name of a not particularly bright fowl is a (deliberate?) coincidence.
The Ottomans called Westerners Franks. All nations and people who were Catholic, or on the wrong side of the Danube, with a penchant for Crusading, were known as Frankish. French, English, Germans, Venetians – were all varieties of Franks. The name persisted even as some of the Franks abandoned Catholicism. Hot dogs are an arguably different variety of Franks.
Both Franks and Turks also called each other infidels. Albeit in their own tongues.