What language is spoken on international space station?

I can't say I'm a big expert on this, but I consume a lot of media from NASA (NASA TV, NASA YouTube channel, etc etc) and it seems to me that the dominant languages on board are English and Russian, but the cosmonauts all seem to be fluent in English. So English seems to be the lingua franca on board. However, I've also seen in some videos cosmonauts conversing in Russian with one another, so it's likely both depending on who is talking to who. Also, it seems to go the other way with US astronauts coming away from a stay on the ISS with some familiarity in Russian.

The current crew of the ISS , as of today (March 16, 2011) is composed of one man from Russia, one man from Italy and a woman from America, so they all speak different native languages! I would expect that English is the dominant language in the current situation since it's generally the language of educated and professional people the world over (I hope that statement isn't interpreted as chauvinistic. It's true.) In addition to Americans, Russians, and Italians, there have also been astronauts from Japan, Canada, South Africa, Germany, Spain, Brazil, Iran, Sweden, Hungary, France, South Korea, Belgium, Netherlands, and Malaysia who have visited the station.

I wonder if one day, the off-world colonies on the Moon, Mars and the satellites of the gas giants will be an English/Russian creole language....