Android/iPhone: Duolingo is easily one of the most popular language learning tools, and today it's been updated with a new club feature where you can pair up with friends to compete at learning new skills.
The new language clubs let you group up with friends, then compare your weekly accomplishments on a leaderboard. There's also a newsfeed that shows what you accomplish in the app, so you can keep an eye on each other's accomplishments much like you would with exercise. If you can't stand learning in a void, this might be the little push you need.
Clubs are available for English, Spanish, Portuguese, French, German, Italian, Russian, Dutch, Hungarian, Ukrainian, Turkish, Korean, Chinese, Japanese, Vietnamese, Indonesian, Greek, Romanian, Czech, Polish, Thai and Hindi.