General Sheridan of the United States Army (famous for supposedly saying "[t]he only good Indian is a dead Indian," which he denied) was accompanying the German Army as a distinguished neutral observer. According to his memoirs, just after the King had received Napoleon's surrender by letter and left for Vendresse,

It was after seven o'clock now, and hence too late to arrange anything more where we were, so further negotiations were deferred till later in the evening, and I, wishing to be conveniently near Bismarck, resolved to take up quarters in Donchery. On our way thither we were met by the County nephew, who assuring us that it would be impossible to find shelter there in the village, as all the houses were filled with wounded Forsyth and I decided to continue on to Chevenges. On the other hand Bismarck-Bohlen bore with him one great, comfort-some excellent brandy. Offering the flask to his uncle, he said: "You've had a hard day of it; won't you refresh yourself ?" The Chancellor, without wasting time to answer, raised the bottle to his lips, exclaiming: "Here's to the unification of Germany," which sentiment the gurgling of an astonishingly long drink seemed to emphasize. The Count then handed the bottle back to his nephew, who, shaking it, ejaculated, "Why, we can't pledge you in return-there is nothing left !" to which came the waggish response, "I beg pardon; it was so dark I couldn't see;" nevertheless there was a little remaining, as I myself can aver. http://www.shsu.edu/~his_ncp/Sedan.html

This exchange took place in English, which was one of the languages that Bismarck spoke well, multilingualism being an occupational necessity for a diplomat in those days. Sheridan in contrast was a simple soldier and did not speak German (..."[my] conversation ...[with the King of Prussia]... was brief, as neither of us spoke the other's native tongue") In fact, Bismarck later joked when replying to a German-American's speech in German that he was pleasantly surprised as he had previously been obliged to address Americans in English only... https://books.google.com/books?id=rukGAQAAIAAJ&pg=PA17

TL;DR: it wasn't a big formal toast, just Bismarck and Sheridan and a couple of others riding along in the dark looking for a house that wasn't in ruins where they could doss down and it was in English because that was the only language they had in common.