In regards to my previous post on Rabbi David Frankel's sermon of thanksgiving after the Prussian victory at Leuthen, S. kindly pointed out that the sermon was likely the product of Frankel's famous student Moses Mendelssohn.
I managed to track this to Alexander Altmann's biography of Mendelssohn. Here is the relevant passages:
No account of Mendelssohn's exercises in ars poetica would be complete without some mention of his synagogal hymns and sermons. The young bel esprit was by no means averse to putting his talents at the service of the Berlin community. The chance to do so was provided by the Seven Years' War. When Austria and Saxony opened hostilities against Prussia toward the end of 1756, the Jews of Berlin added to their daily prayers the recital of certain appropriate psalms and a special prayer composed in Hebrew by Hartog Leo and translated into German by Mendelssohn. Frederick II's surprising victory at Rossbach caused great jubilation and was celebrated by a thanksgiving service in the synagogue on November 12, 1757. Mendelssohn, again, translated a Hebrew text, a hymn written by Hartog Leo, into German. It was published by the community, and it seems that it has also been planned to publish Mendelssohn's German version of a sermon preached by Chief Rabbi Frankel. Another great victory, at Leuthen, was duly celebrated on December 10. 1757. The same pattern was repeated, but this time both the hymn and the sermon were published in German. According to the title page, the sermon had been "delivered" by Frankel and then "translated into German" (omitting Mendelssohn's name). In fact, however, Mendelssohn had written the sermon, as he remarked in a letter to Lessing that, on internal evidence, can be dated about December 15, 1757: "I shall no longer swear to anything in the world after it has come to pass that I write a sermon and praise a king. I also translated some Hebrew thanksgiving hymns into German, and these are printed." This sermon, which is the one praised in Lessing's letter of December, 1757, is the earliest known specimen of modern Jewish preaching in the German tongue. It has a slightly philosophical flavor and reflects the spirit of the Enlightenment. (Moses Mendelssohn: A Biographical Study pg. 67-68.)
So Mendelssohn authored the sermon and translated it into German. This still leaves the question of who translated the sermon into English and how did it come to be published in London and in New York?