Which user on Duolingo has the most amount of coins/levels? just curious...
The highest achievable level currently is 25 in any language. After that point you can continue to collect points, but are at the max level. There is no way that I am aware of to search for users by level/points. I do know there are users that have reached level 25 in at least one language however and users with 30K+ points.
I would say 11 or 12 is the highest level attainable without translating a document. Just to make sure, I decided to confirm this hypothesis and went on to do the placement test and redeeming tests in Spanish to obtained the highest level possible. I graduated or finished - in a couple of hours- the tree for this language (practically my other mother tongue) losing five or six hearts just because of typos and never reached level 12 (you surely guess that my word counting is comparatively low). So I would say a realistic target should be level 12 to 13. But hey, it is about learning not reaching levels!
Levels depend on the amount of coins, not on actual knowledge or position in the skill tree. You can finish the tree being level 11 or 12 if you go really fast and revise very little, but there are users who are level 20+ when they complete the tree. I finished my French tree being level 21 and my German tree being level 22. This is because German was harder to me and I had to practice it more. I did some translations, but not really many.
Also, as far as I know, levels don't continue after 25. You still get coins but your level remains 25.