Boswell tells us that, on June 6, 1764, he hired a Bernois named Jacob Hänni to be his servant for the Grand Tour, and indeed they did spend most of the next year and a half in each others company. He spoke French and German.
The first name Jacob and surname Hanni (or Hänni/Haenni) were particularly ordinary in the Bern area in the 17th and 18th centuries, so it has been hitherto impossible - for me, at least - to identify Boswell's servant with any certainty.
Life with Boswell:
Boswell hired Hänni on June 6, 1764, to be his servant for the duration of his Grand Tour, which began a few days later. Although not as considerate and kind as Boswell's former servant, Francois Mazerac, and despite occassional rows, Boswell was quite satisfied with Hänni, due to his usefulness, intellect and common sense. On July 25, 1764, Boswell describes him as "a genteel, active fellow. I liked this specimen of him upon our journey to Potsdam from Utrecht. He was always alert and ready to put everything right. [...] He is quite sober, has good Christian principles and even generous sentiments. He would fight for his master, and gold could not tempt him to marry a woman he did not like."