Level 25 is the highest and it takes 30,000XP to reach it.

Highest fluency % appears to vary depending on which course you are taking. Also, it keeps getting adjusted as staff tinkers with it, so all I can say is that it is usually between 50 and 70%. However, it doesn't measure fluency in the outside world, only how you are doing in the Duolingo course. Though, even that keeps getting tinkered with. So, most people here just consider it a kind of quirky thing that can be fun to watch increase, but not something to take very seriously.

The highest levels do fluctuate by language. I'm at 69% in French (studying beyond level 25) and 77% in English (I'm a native speaker) at level 16 in a "reverse tree". I saw the other day someone said he was at 72% in French. That is the highest I have heard of for French. Note: these fluency levels are more about course proficiency, not real world proficiency. To really learn, you need to work on listening and speaking skills. Work on verb conjugation at listeningpractice.org. Try translating simple books or newpaper articles. Google translate is your friend. Use it to compare your translations. Speak as much as you can out loud, not just in your head - your tongue needs the work.

Level 25 is the highest (you need 30,000 XP). As for the fluency%, although it is irrelevant, the max is about about 65-68%.

Thanks! I've got just under 20 000XP to go then! ;)