If you feel the need to act pretentious, or merely wish to communicate the reference to someone brought up on the original French versions of the Asterix books, the canonical form of this line is "Ils sont fous, ces romains". Furthermore, the finger is not circled in the air, but tapped against the forehead, producing the sound effect "Toc! Toc! Toc!". The Italian translation uses an lightly modified version of a common slur directed by other Italians against the inhabitants of their capital city, "Sono Pazzi, Questi Romani" (initial capitals are obligatory, of course).