I don't think the answer is on the site anywhere, it's a bit of a strange issue but it's by no means unique.
Whilst MNFF technically holds some rights to your fiction, in the fact that you've agreed to place it on our site, you are the author and I'm sure no one here will restrict you from consenting to it being used elsewhere. Many authors publish their stories on many fanfiction sites so it would seem a bit hypocritical of us not to let you. The danger lies only in the fact that your story will be translated into a language you do not understand, so you cannot validate it and you may be putting your name to something that doesn't closely resemble your original story. That or your translator may claim your work as his. These are only small warnings though, I'm sure most of these people just want to pass on the fantastic stories they read to people who are less comfortable reading English. I just ask you to be careful and prepared.
As far as MNFF's rights, I think we would claim a right to your story only if we had to move our database for some reason, and that would be only a basic right which the author would be able to deny and take down their fiction if they did not want it to be wherever we had placed it.