'Lisp' isn't entirely accurate , which is why I put it in quotes, but there certainly is Castillian influence on the language: http://en.m.wikipedia.org/wiki/Rioplatense_Spanish
I've been to BsAs three times and once to Carmelo in Uruguay, a few minutes drive from Fray Bento. Rioplatense is definitely distinct from more common dialects of Spanish and might be picked up as a bit of a lisp.