It was pretty much the same as XI. You'd find a nice one every once in awhile who you let you join in on their parties, but most of the time it was jp only. Some of my LS buddies were regularly invited by jp parties, but only b/c they knew enough japanese to communicate with them and/or used some translator on the internet. Other than that, as far as I know, there really wasn't very much intermingling.
They really haven't announced how many or where the international servers will be, but I've always heard that there will probably only be a server in Los Angeles to act as the server in the US. I'm not sure if they'll have an east coast server at all. They only thing they've really confirmed at this point is that they'll have data centers in North America. If I were a betting man, I'd bet money that most people with choose the servers that are closest to them to reduce any lag they might encounter. If this is true, it's going to hurt the opportunities for people to interact with others from other regions around the world. Of course, they are not forcing you to join any particular server unless you are going to use a 1.0 character. If you start a new character, you can join any server you want, just know that if it's associated with a data center outside of your region, you may encounter more lag than other that live within that region.