Everything Nobby said was spot on.
Whatever kind of wedding it is, you can't really go wrong by wearing a hat. It might not be compulosory, but it's never frowned upon either. Don't wear a red dress, or something which could "upstage" the bride. It's always nice to bring a gift of course, though I wouldn't reccomend wine since you'll be in wine country and might be seen as a slight insult. Other alcohol would be just fine, though. Other than that, weddings are usually pretty smart affairs, so a nice dress should be in order.
Other than that, nothing special. They do drag on for a bit, church service in the morning and then an endless late lunch/dinner would be my guess.
You should really go, though. Bordeaux is lovely, the region is gorgeous, the food and the wine are out of this world, and drunk french people are usually pretty nice and friendly. And, if we're honest, any faux-pas you might make will be forgiven by the simple fact you`re American, and therefore not supposed to know the local customs.
As for the language, "Enchantee!" when you meet/are introduced to someone is always good. Most people will speak enlgish though, the more so the drunker they get. Finally, a beautiful woman is always warmly welcomed and very appreciated in France, so don't worry too much, you'll be just fine, and people will make sure you're ok.
If you want me to find a hotel, PM me the details of where the wedding is and your budget, and I'll do a quick search to see what I can find, and send you a pot-pourri of my findings.