Duolingo

Duolingo is an American educational technology company which produces language-learning apps and provides language certification. On its main app users can practice vocabulary, grammar, and pronunciation using spaced repetition.

"Fluent" is a misleading way to measure how well you know a language, because it implies there is an endpoint to learning it. In fact, there's no test or language criteria for deciding if someone is "fluent," and language learning experts instead talk about proficiency.