Arthur Gerald Wellesley, Earl of Mornington, is the great-great-grandson of what  last German Emperor ("Kaiser") and King of Prussia?
Wilhelm II, German Emperor Wilhelm II or William II (German: "Friedrich Wilhelm Viktor Albert von Preußen", English: Frederick William Victor Albert of Prussia; 27 January 18594 June 1941) was the last German Emperor ("Kaiser") and King of Prussia, ruling the German Empire and the Kingdom of Prussia from 15 June 1888 to 9 November 1918. He was the eldest grandchild of the British Queen Victoria and related to many monarchs and princes of Europe.
Duc de Dalberg The title of Duc de Dalberg was created by the French Emperor Napoleon I on 14 April 1810 for Emmerich von Dalberg, the nephew of Karl Theodor von Dalberg, Prince-Primate of the Confederation of the Rhine and Grand Duke of Frankfurt. He died on 27 April 1833. His daughter and heiress married firstly Sir Richard Acton, 7th Baronet (by whom she was the mother of John Dalberg-Acton, 1st Baron Acton) and secondly Granville George Leveson-Gower, 2nd Earl Granville, but as the Duke had no sons, the title became extinct.