Unfortunately, this is a feature that was available through source code hacking, but is currently not possible, because they disabled that feature. I believe it will be available at some point, but doesn't seem to be on the road map any time soon. I have been fighting this for a few months, so your best bet is, like me, pester the Ning staff and let them know that we want user language selection!
I'm working on a temporary dirty hack, but if I make progress, I'll be sure to post here and let you know!
The weirdest thing is that you can change language from the backend - the functionality is already there. I don't understand how it could be so hard to allow the user to select the language instead of the administrator?