Translation:I use Duolingo to learn Chinese.
Antonio "studying a language" has a whole different meaning than "learning a language". Studying a language can refer to linguists learning everything about the history of the language, it's origin, culture and how it evolved. Learning a language on the other hand.. Is just knowing how to use the language, nothing more. Learning Chinese is not the same as learning about China or the Chinese people though studying Chinese could mean that.
I agree with Ankichikun. Consider another example: My Chinese students often say "I learnt English this morning". This is absurd; it implies completion. You could say "I learnt English in school (and now I'm fluent)" but it isn't a task of two hours. You could say "I learnt (about) the present perfect" or tag questions, but not the entire language. What is meant is "I studied English this morning." In this sentence the difference isn't so stark, but I'd still prefer study over learn.