How I Use It
I used Duolingo in Middle School with several grade levels in French (Foreign Language)
I used this to make students practice structures and vocabulary in French.
What worked: students learnt new words, revised some gramatical content.
What didn't work: Because it relies on translation, this app is limited as sometimes the content cannot be 'literally" translated and students loose points because of the way the app expect the translation in English. It is also confusing for students to learn the word genders (feminine and masculine) because it does not exist in English so they don't think in the target language, they simply keep translating in English which does not make them think in the target language and conceptualize.
This tool is good for basic use OR for advanced learners who are aware of the "translation" problems and are still willing to use this app despite the problems they might encounter due to literal translation.
I would suggest "Rosetta Stone" instead, which never relies on translation, but this product is expensive.