Suggestion about the voices and a possible upgrade (written by a Brazilian).
Again, I saw a discussion that gave me an ideia. The discussion was to swap the robotic voices and use native people's voices. Then, one guy said at this discussion that DuoLingo already made a test and people usually prefer the robotic voices. My idea: create a space where users could share videos and audios like learning musics, or studing videos. Perhaps, Duolingo could create this midia. Of course its a difficult thing, it would take a while, but it's a nice thing to do and it would make to look more familiar other languages.