I have to ask - what's in it for them? Are they devoted fans who just want to help? Will the community bow down before them and thank them to the heavens?
I'm not saying people need an obvious (i.e. financial) benefit, but they need to at least get a really good feeling from it.
So I would ask
1. What are you doing to make them feel like they will get profoundly thanked?
2. How are you making sure you deliver on that promise?
For example, do you have a list of active translators? If it's a forum or place where user names show up in the normal course of things, do they get a "Community Superhero" designation or something like that?