Three essential elements of Duolingo learning are that the site is crowdsourced, learning is gamified, and people whose language is not English can learn another language. The most robust second (or third) language learning opportunities are available to speakers of English, Spanish, Russian, French, Portugese, and several other languages. Speakers of Hindi, Czech, and Polish, to name just a few may learn only English. At the moment; the company wants to expand offerings.
There's also Duolingo for Schools. Parents can use it to set parental controls (disabling certain vocabulary words and social interactions).