Overall top translators for a specific language
I've noticed that on the document translation page, it has a list of the top translators for that document. That may be another way to gauge the credibility of a translator, when in doubt between two translations to be rated.
But regarding motivation, I think it would be more meaningful to show the top translators for that given language.
There could be a general ranking for several things like number of sentences translated or skill points per languages. Such ranking could be for everybody to see. And you could see your position into it. I think it would be motivational. But i want it probabily cause of my curiostity. So the site could have both ranking of translators by document in the document page and general ranking in it's own page.
Now that I think of it, the overall ranking is also more useful in assessing the translator's overall credibility. What the document ranking theoretically shows is which translator has the best understanding of the context of the document (assuming a more or less similar number of translated sentences per translator).