"The first tongue, the mother tongue, spoken by Adam, Sem, and Noe, was different, and it is now extant only in isolated dialects. Its first pure offshoots are the Zend, the sacred tongue of India, and the language of the Bactrians. In those languages, words may be found exactly similar to the Low German of my native place. The book that I see in modern Ctesiphon, on the Tigris, is written in that language."
More details, including the alphabet and appropriate PIE stage: http://www.wikinfo.org/index.php/Adamic_language