The official answer is “Adamic,” which is the language that was spoken until the Tower of Babel was built and God rewarded that bit of folly by giving people different languages.
(Oh, and if you’ll accept the literal truth of God making Adam from clay and Eve from one of Adam’s ribs, you should have no problem accepting that God made them both so that they could understand Adamic.)
I’m seconding telepathy, since I keep hearing that humans were capable of doing it before language (and words and symbols) became an issue (supposedly there’s still a tribe in Africa that communicate that way for when their hunters go out.) Snakes can’t make too many vocalizations anyway, and that’s how paranormal beings (the devil?) do it in accounts all the time.