You could hard code this in comments.php but you'd have to re-do this with each update.
Alternatively look for an Italian language file it_IT.mo in
http://forum.bytesforall.com/forumdisplay.php?f=11 and put that into the theme's main directory, that should work automatically from there. Or, create your own language file with the POEdit software and the base language file atahualpa.pot (in the theme's main directory)