I see your winky, but I'm keeping my serious hat on this time, so pardon me if I sound stubborn (well, I am).

Isn't this view of culture kinda... I dunno, outdated, for lack of a better term? Nowadays culture is global. Stuff goes on the internet, everybody sees it. I think one should be prepared for that. And this is a show that relies its entire existence on pop culture and nerds! Nerds! People that for definition are the most anal retentive on the face of the earth! They're gonna notice the error, they're not gonna be pleased.
"The Simpsons" is guilty of this, too. In that horrible episode in which Sideshow Bob is somehow the Mayor (oh, sorry, Mayore.... yuck!) of a town in Tuscany, none of the italian spoken is even close to correct, it sounds like it was translated with Babelfish.
Getting back to Robot Chicken, I understand the general view of Opera is "people yelling in an incomprehensible language", but try to see this from the other side.

Picture some skit on italian television that tries to parody a Broadway show, and has all the characters speaking and singing in incredibly mangled english. Wouldn't you be baffled, at least? There's clearly an effort in getting people that can sing, dressing them up in beautiful costumes, having them do elaborate choreographies, and then they fall flat by having them sing stuff that sounds like it was taken from "Backstroke of the West".

Bottom line, either have foreign characters speak/sing complete gibberish, or spend a couple of bucks more and pay somebody to translate things properly. Machine translations do not work! Especially with Dante's language!