Question: Dorothea von Salviati's father-in-law was born on what date?

Context paragraphs:
Princess Dorothea of Courland: Dorothea von Biron, Princess of Courland, Duchess of Dino, Talleyrand, and Sagan, known as Dorothée de Courlande or Dorothée de Dino (21 August 1793 – 19 September 1862), was a Baltic German noblewoman.  Her mother was Dorothea von Medem, Duchess of Courland, and although her mother's husband, Duke Peter von Biron, acknowledged her as his own, her true father might be Polish statesman Aleksander Batowski.  For a long time, she accompanied the French statesman Charles Maurice de Talleyrand-Périgord, whereas she was the separated wife of his nephew, Edmond de Talleyrand-Périgord.

Dorothea von Stetten Art Award: The Dorothea von Stetten Art Award (German: Dorothea von Stetten Kunstpreis) has been awarded biennually since 1984 to an artist under the age of 36 whose work demonstrates interesting perspective and possibilities for advancement.

Dorothea de Ficquelmont: Dorothea "Dolly" de Ficquelmont (14 October 1804, Saint Petersburg, Russian Empire10 April 1863, Venice, Austrian Empire), born Countess Dorothea von Tiesenhausen ("Daria Fyodorovna" in Russian), was a Russian writer and salonist.  A granddaughter of the Russian war hero General Prince Kutuzov (who distinguished himself in the Napoleonic Wars), she was a Russian aristocrat of German Baltic origin, and later a member of the Austrian nobility as the wife of Count Karl Ludwig von Ficquelmont.

Dorothea of Montau: Saint Dorothea (or Dorothy) of Montau (German: "Dorothea von Montau" ; Polish: "Dorota z Mątowów" ) (6 February 1347 – 25 June 1394) was a hermitess and visionary of 14th century Germany.  After centuries of veneration in Central Europe, she was canonized in 1976.

Dorothea Wyss: Dorothea Wyss (* around 1430/32 in Obwalden; † after 1487 in Sachseln), also known Dorothea von Flüe, married Niklaus von Flüe, the patron saint of Switzerland.

Dorothea von Medem: (Anna Charlotte) Dorothea von Medem (3 February 1761 – 20 August 1821) was born a Gräfin (Countess) of the noble German Baltic Medem family and later became Duchess of Courland (a Baltic region).  Popularly known as Dorothea of Courland after her marriage to Peter von Biron, the last Duke of Courland, she hosted an aristocratic salon in Berlin and performed various diplomatic duties on behalf of her estranged husband.

Wilhelm, German Crown Prince: Wilhelm, German Crown Prince (German: "Kronprinz Wilhelm von Preußen" ; 6 May 1882 – 20 July 1951) , full name Friedrich Wilhelm Victor August Ernst, was the eldest child of the future German Emperor Wilhelm II and his wife Empress Augusta Victoria, and the last Crown Prince of the German Empire and the Kingdom of Prussia.  After the death of his grandfather Emperor Frederick III, Wilhelm became crown prince at the age of six, retaining that title for more than 30 years until the fall of the empire on 5 November 1918.  During World War I, he commanded the 5th Army from 1914 to 1916 and was commander of Army Group German Crown Prince for the remainder of the war.  Crown Prince Wilhelm became Head of the House of Hohenzollern on 4 June 1941 following the death of his father and held the position until his own death on 20 July 1951.

Dorothea von Salviati: Dorothea von Salviati (1907–1972), was the wife of Prince Wilhelm of Prussia, the eldest son of Crown Prince Wilhelm, the eldest son and heir of the last German Emperor, Wilhelm II.

Dorothea of Saxony: Dorothea of Saxony (German: "Dorothea von Sachsen" ; 4 October 1563, Dresden – 13 February 1587, Wolfenbüttel) was a Saxon princess from the House of Wettin and by marriage Duchess of Brunswick-Wolfenbüttel.

Dorothea von Ertmann: Dorothea von Ertmann (born Dorothea Graumann, 3 May 1781 – 16 March 1849) was a German pianist.

Supporting paragraphs: Dorothea von Salviati Wilhelm, German Crown Prince
Answer: 6 May 1882