One should not confuse languages with dialects and divide between written language and spoken word. At least since Martin Luthers Bible translation and the reform of charter language (German instead of Latin) , a standard German written language had been established. In the 18th Century, people like Lessing, Goethe or Schiller wrote the purest form of German, which is still the standard German of today (except some (lesser) grammatic and othrographic changes). Of course, Goethe, who was born and raised in Frankfurt, at home, may have spoken some "Hessisch" dialect (no audio tape exists), but at Weimar, where he was part of the government, he certainly spoke High German.
At the courts, the lingustic situation was quite international. French was the official diplomatic and courtly conversation language. Friedrich the Great wrote almost all his works in French, except a few German texts, and discussed things in French with Voltaire and other philosophers of the Enlightment. Same did Catherine the Great (from German (Anhalt) heritage) in Russia. I think, the unlucky Marie Antoinette spoke fluently French already at Schönbrunn, even before she arrived at Versailles. The famous Vienna Congress of 1815 was held in French. Mozart used a French text (Beaumarchais) for his Figaro opera (libretto in Italian). Mozart lived and studied in Italy for more than 3 years, and he spoke Italian fluently. Italian was the standard language for Operas, his Vienna rival Salieri was an Italian. He was firm in Latin, too. But his native language was German, maybe he used some Vienna or Salzburg dialect at home, again we have no audio tapes. His letters are written in Italian or high German.
At the courts, the lingustic situation was quite international. French was the official diplomatic and courtly conversation language. Friedrich the Great wrote almost all his works in French, except a few German texts, and discussed things in French with Voltaire and other philosophers of the Enlightment. Same did Catherine the Great (from German (Anhalt) heritage) in Russia. I think, the unlucky Marie Antoinette spoke fluently French already at Schönbrunn, even before she arrived at Versailles. The famous Vienna Congress of 1815 was held in French. Mozart used a French text (Beaumarchais) for his Figaro opera (libretto in Italian). Mozart lived and studied in Italy for more than 3 years, and he spoke Italian fluently. Italian was the standard language for Operas, his Vienna rival Salieri was an Italian. He was firm in Latin, too. But his native language was German, maybe he used some Vienna or Salzburg dialect at home, again we have no audio tapes. His letters are written in Italian or high German.