If you haven’t already, I’d strongly urge you to read a fantastic series of blog posts by Julie McDonough-Dolmaya at http://mcdonough-dolmaya.ca/2011/ . She surveyed volunteer translators and what she found was fascinating. I’ll give you a quick preview in this chart:
Translator motivations (click to view)
The whole series of posts is excellent; I encourage you to read them.
pb