She loves Harry Potter so much that after reading the whole series in Romanian, she started to read the first volume in English. Didn't finish that either, but managed to read about 100 pages of it and she said that after 30-40 pages, it became substantially easier to understand.
Now she's among top 3 students, if not the first, in her class at English, outpacing colleagues who take 2-3 hours weekly of extra English lessons, without having a single paid English lesson.
Duolingo might not teach you how to speak a language fluently, but it definitely can give you a headstart in learning one.
She's not able to speak English fluently yet, but she's able to watch TV series without subtitles and she's able to actually communicate with other English speaking people when visiting other countries.