Europa Fleet fits your description, while we aren't one of the multi-national - an active group of players all day, all night sort of thing, we always have something going on, whether it's PvP or helping out a fellow member with his mission.
hi at the 4th fleet we are mature, structured, and active. we are based in the usa est. but we have members from all over the world. sweden, england, greece, etc, there is always someone on our team speak to talk to or play with. that is one of the must is to have team speak as that is one of the ways we determine your activity. our new member are required to participate in the replacement academy. it is easy and helps you learn about the fleet and help you enjoy the game more. you can apply at http://forums.4th-fleet.com/index.php you can read our history, structure, and what is expected from our member at http://forums.startrekonline.com/showthread.php?t=88444
We are mature, we are medium RP but don't require that of our members. We have lots of stuff for members' use, but our basic affiliation requirements are that you share knowledge (as in game tips), and participate with us (as in teaming on missions) - everything else we have and do is optional. Our Fleet exists to enrich the gameplay of our members, period. We have members from all over Earth, and of all different playing styles and types, you're sure to find plenty of commonality to run missions. For me, there's nothing like the fun of doing missions with other Fleet-mates who let me use my strengths and skills to help them, and I can depend on them for their strengths and skills - whilst coordinating our action to accomplish the mission with aplomb and flair.
Good luck on your hunt for the right Fleet, it would be great if it was the VKF!