to all the sicilians here, how accuratly is the sicilian language portraied in this movie (especially by the non-native speakers)? also, how valid is the portrayal of sicilian costums, are they really correctly done in the godfather trilogy?

As an Italian (but not Sicilian), I'd say that the portrayal of Sicilian customs is very accurate, especially in the Sicilian settings. As for the language, I can just say that DeNiro's almost completely Italian acting in GF2 was absolutely great, if you consider that he is NOT mother tongue. The occasional Italian words one can hear in the trilogy by non Italian speakers sound very "American" (if you know what I mean) but I presume sounding very Italian (or Sicilian) would not be realistic.

what i meant was, did it sound particular sicilian? also the italian actors were not all sicilian, for instance the appollonia actress was born in rome. did you hear any inconsitance in the accents?

also, i wondered, when young vito talks to senior roberto, he asks him whether he's sicilian. wouldn't he know by the way he speaks, that he's calabrese?

no, I think they all tried to sound Sicilian. It's very easy to imitate the accent. I can do it! Gastone Moschin, playing Fanucci, for example was from Veneto (the region of Venice, up in the North), but he sounded Neapolitan -- in fact, if I remember well Fanucci was Neapolitan. Anyway, Moschin did not sound Venetian, even though his natural voice definitely has a Nordish accent.

OMG, you're more meticulous than I!!! Consider that Vito left Italy at a very young age, maybe he was not familiar anymore with Italian accents.....

ok, maybe i should have phrased this different! wouldn't he know that he's not sicilian?

As a Sicilian-American, I assure you that the dialect spoken is very accurate.

As for young Vito's encounter with the landlord, I think perhaps he was trying to establish a common bond with Roberto by asking him his origins.

In the film, young Vito probably knew Sr. Roberto wasn't Sicilian--if not by his accent, by his reputation in the neighborhood. As a landlord, everyone would know his background ("...that no-good Calabrese...").
DeNiro spent months in Sicily learning how to speak like a Sicilian.