I'm finishing to write a spec. Since it's set in an imaginary town wich could be american, and since I think it's much more suitable for the american film industry rather than the italian, I'm thinking about making translate and try to sell it in the States.
What I need to know is:
1) is there some law's restriction for people who are not american? I mean something like "I can't have a movie credit [if the movie was made] 'cause I'm foreigner"... or something like that?
Not in the US industry. The US industry is the only industry where there is no government support, therefore there are no nationality restrictions. You need a green card to work in the biz, but you don't need anything to sell a script.
(2) do you think that after I got it translated by a professional italian-american screenplays' translator (whom I can find also in my hometown) I can find in L.A. some reliable person to improve it fixing the language and the dialog? are they very expensive? usually do they demand a movie credit? And - most important - how can I find them?
Translating a screenplay usually runs a couple thousand bucks. If someone's just polishing the language, maybe a thousand bucks. If you're asking the US writer (a Canadian would do just as well, really) to adapt
the script into something more American in tone and style, though, you're looking at more money -- essentially at that point you're asking for a polish or even a rewrite. Then you're looking at more money and (in the case of a rewrite) potential credit.
(The Italian proverb above means roughly, "Translator, traitor, what's the diff?")