It's a French/Italian wedding with (presumably) friends from Switzerland. Just in that one sentence, there are a million and one variables, and you are not going to be able to predict them all. The wedding of a bride from an artsy Milanese family to a Parisien from the 5th arrondissement is going to be very different from the wedding of a bride from rural Puglia to a banker from Grenoble.
The good news is that all the guests are going to be equally confused, and I reckon you can get away with almost anything.
That said, though the green silk dress is gorgeous, I agree with everyone warning about sweat stains. Italy can be a bastard for humidity, even at night. A crotch sweat mark is not a chic look. A navy or black silk would hide sweat better. Also, don't forget mosquitoes which can be a nightmare. Whatever you wear, take an emergency cover-up wrap, in case they are biting.
Some of it also comes down to how bothered you are about blending in. The French are competitive under-dressers. Their aim is to look amazing but also as if they haven't made too much effort. So they will probably be in something less evening-y than what you have in mind, But does it really matter if you are dressed differently from the majority, in such an international crowd? It sounds like a fabulous occasion. Go with something that makes you feel amazing (definitely not a polyester jump suit), and enjoy!