Big Grammar Error!
Well just see this pretty much: http://codegolf.stackexchange.com/questions/66841/youre-on-a-8-day-streak Guys, this is sad, its an app for teaching language, and it has a huge grammatical mistake. When you are on a streak, and the number starts with a vowel, it does not appropriatly handle an vs a.
I don't think it is that big an issue. (You should see the German duolingo introduction page ...)
Also, it could be solved quite easily without adding any extra code by simply rewording the sentence like this: "You are on a streak of xx days!"
(As a programmer, I do appreciate the challenge, though.)