French

 One recalls that War and Peace begins with paragraphs of "that accurate French in which our grandparents spoke and even thought". I had not perhaps realized how far French served, until yesterday I encountered a paragraph by Peter Demetz on Maria Theresa:

The dynasty was her nation; she corresponded with her children in French; as for her German, she spoke it with the sophistication of a plebeian Vienna wet nurse, as a popular ditty of her time suggested, and wrote the language of Klopstock and Lessing quirkily and according to French syntactical rules (only Frederick of Prussia's German was worse, but he was, after all, a French writer of note).

 (Prague in Black and Gold: Scenes from the Life of  a European City, chapter 6, "Mozart in Prague")

I had read that Frederick preferred French. I have not encountered his writings, and would be no judge of his French if I had, but Samuel Johnson thought poorly of his work:

 Sir Thomas [Robinson] said, that the king of Prussia valued himself upon
three things;--upon being a hero, a musician, and an authour. JOHNSON.
'Pretty well, Sir, for one man. As to his being an authour, I have not
looked at his poetry; but his prose is poor stuff. He writes just as you
might suppose Voltaire's footboy to do, who has been his amanuensis.
He has such parts as the valet might have, and about as much of the
colouring of the style as might be got by transcribing his works.'

 (Life of Johnson, entry for July 18, 1763)