Anecdata, but I completed the whole french duolingo tree and kept my whole tree "gold" the whole time, and I still don't feel like I'm proficient in french at all. I thing duolingo can be a good addition to another language learning system, but on its own I think it's next to useless
I'm the same with French. A few years later I recall nothing. I am proficient with Japanese which I learned using traditional methods and lots of reading over a few years. There's no magical easy way to learn a language!
