Mithridates, 1806 I finally heard why the book is called Mithridates.
The Mithridates I wrote about is Johann Christoph Adelung's compilation of 450 paternosters in all known languages, Mithridates oder: allgemeine Sprachenkunde mit dem Vater Unser als Sprachprobe in beynahe fünfhundert Sprachen und Mundarten, 4 vols. Vossische Buchhandlung, Berlin 1806-1819.
Today Toon van Hal, historian of linguistics at the University of Leuven, informed me that the title refers back to an earlier book of the same name, Mithridates de differentis linguis (1555) by Conrad Gessner, which described 130 languages of which 22 were presented by a sample of the Paternoster. This again refers back to the mythically polyglot King Mithridates VI of Pontus (132-63 BCE), formidable opponent to the Roman expansion, who is said to have spoken the languages of all the 22 nations he governed.
[probably the wiki lemma about Mithridates has been updated since - I remember consulting it back then.]