I wouldn't call it a direct translation, though Halleck was definitely one of the ones who read Jomini (and he even refers to Clausewitz in at least one place-- Clausewitz didn't attain his present level of appreciation until later). Halleck's principal translation was actually of a biography of Jomini, rather than Jomini's Art of War itself, although I do wonder if the 1862 West Point translation was influenced by Halleck... anyway, Halleck was unquestionably knowledgeable of Jomini, and to an extent that was not normal among his contemporaries, West Pointers or otherwise.