I know they are seperate things; if I didn't, then my post would lack any internal logic.
I am saying that turning Selina into- for example- a black American, would be a less significant change than turning her into a Frenchwoman.
I'm not really advocating changing her nationality, but if Nolan was
so impressed by Cotillard's performance in
Inception, what's saying he might not want to write the part for her? It's not unheard of for writers to do that. Again, this character will have to be
adapted to the series (as this isn't a comic book, nor is it the same continuity), meaning it will most likely not be 100% true to its comic counterpart in a few or more senses.
Personally, unless she has ties to Falcone in this series, I don't really care whether or not she's Italian. I mean, I can't judge what I'd want her to be
exactly, since we don't know what story we'd get out of Catwoman. If we get a bastard child looking for her father (Falcone), then yeah, it may make a little more sense to delve into Italian roots. If we just get a gorgeous socialite named Selina Kyle who is a fantastic thief and- coincidentally- a life-changing courter for Bruce, what does her nationality matter? Besides, Kyle isn't even an Italian last name (nor is it French, it's derived from the Scots).
I'm more concerned with her attitude and her interactions with Bruce and her opponents than I am with what her nationality- or even her name- is.
That's where her character lies.
All that said, if the writing for the character calls for an American accent, great. Make the actress learn an American accent. It's not a big deal to me. I just love Cotillard's accent, and wouldn't mind it one bit if she used it as Selina.
I didn't mean the actor I meant the character. 
