I've got a friend who works for the ESA in Germany, I visited him at work last year and I think he uses only German and English, even if there are scientists from many different countrys there.
I don't know how good the chinese space program allready is, but I would prefer German or Russian. And after all the two languages are sure more easy to learn than Chinese.
For my selve I speak German, because I am a German
and just started learning Russian and I must say, that I like the language, even if you have to learn a new writing to, but thats the same in Chinese.