[piffey]

Sorry didn't see this reply or would've commented sooner. So you certainly would need some sort of structured instructions. And there are specific methods for teaching specific languages to people from certain languages. Where the rapid compounding happens is in listening to the native tutor speak. The speaking/listening comprehension portion of language learning is the hardest. When you do pattern matching with Duolingo you're missing out on some of that vital ear training that happens listening to native speakers -- that's why immersion works so well.

So I find it's still best to find someone who is a native speaker, has a language teaching certificate/degree, and to go from there. They'll spot your weaknesses and help you navigate them all while providing a customized ramp, for your learning speed, of all the language's components. Duolingo might spend two weeks on a verb tense that you understood right away or fly over some pronouns that you struggled integrating. This whole time you'll be picking up word choice, intonation, rhythm of speaking from the native tutor that will expedite your usage of the language conversationally.