Background: English first language, French second, and I've been trying to learn Japanese.
The three phases are:
1. Learning basic alphabet (if different from your own), basic grammar. Unlocks: being able to access more learning material and more rapid progress. Progress on these first building blocks is slow and can be discouraging.
2. Learning full grammar control. Even the most basic prose uses ~all of the tenses and conjugation rules. Unlocks: reading/watching non-educational native material.
3. Learning the culture and filling out vocab. Lots of languages are idiomatic, and understanding real-world usage can require just as much that you read Candide as that you memorize flashcards of more-obscure vocab (more-obscure meaning things like "chandelier". It's not an uncommon word, but a second-language learner probably won't run into it in wordbanks or anything.)
I think Duolingo is good for doing 1, and like, half of 2. I don't think it can get you to 3.