Why not identify and rank learners by areas of expertise (e.g., cooking, movies, etc.) based on their translation experience and success therein?
Your areas of expertise would appear on your profile and you could also obtain rankings for an area of expertise.
Note that someone ranked highly in cooking say would just mean that they are good at translating cooking documents and not necessarily that they can cook well.
I think identifying and ranking learners by areas of expertise would provide motivation to translate more by making the process more fun.