Trust n believe the reason i joined Duolingo was to learn new languages, now i don't know when that reason faded, but lately it has become a game. I simply come to gain points so i can surpass strangers! Pathetic and sad but it's the truth!lol
Pride and mastery are good motivators. Duolingo has picked a gamification model for a reason. Fb even uses these models to encourage that people stay on their site.
There is nothing wrong with your motives. The result of your actions are very constructive, you are learning a new language and you are translating the web! :)