Leaderboard is not language specific.
The score shown is actually the maximum obtained across all languages being learned by each person.
@ SuzyStudySpanish - I don't agree. I was top of my Friends leaderboard until a few days ago. Suddenly, one of my friends, who is also studying German and Spanish has twice as many skill points and is now top of my leaderboard (not that I care much).
Also, I've noticed if you hover over the coins, you get a breakdown of the skill points for each language.
@ 1km - But what if it'd had been the other way around? If you had been your friend and your friend had been you? (Good. ^^)
Oh! I hadn't noticed that myself yet.. Well, in that case, you can still easily compare your points for each language separately, or all together, however you please. :3