I knew from the beginning that Duolingo would not take me to fluency in any language I chose to learn using it. I did like that they aligned their European languages with CEFR (Common European Framework of Reference for Languages).
I have used other material. The best one I’ve found so far is Language Transfer. It only has a few languages. I’ve gone through the #Spanish lessons twice and have started on the #German lessons. It’s all audio.
I have also bought graded readers. Dino Lernt Deutsch is excellent. You can find books that will help learn languages through reading stories. Olly Richard’s has a number of books at different levels from beginning to intermediate. He even has his own learning program (I am not using it).
The trick is to read extensively. You don’t have to understand every word. Just get the gist of it. Chapters will include new vocabulary. You can reread the chapters.
Reading intensely means understanding every word. It’s slow and can be boring. You want to read for enjoyment. Eventually you’ll pick up grammar and vocabulary by osmosis. In some ways it’s how you learned your native language. You can study grammar if you need to once you have a better grasp.
You can also watch foreign language tv shows and films. I do with the target language subtitles on.
I don’t speak any languages I’m learning fluently, but I am learning and I hope to be someday.