Updated: Aug 10, 2020
Duolingo was the first free language-learning app that rivaled paid programs. It offers plenty of exercises to build an understanding of dozens of languages or review one you already know. If you're learning Spanish as an English speaker, you'll find a podcast, interactive short stories, and even get-togethers with other learners (which are online-only in light of COVID-19). More boutique languages don't have as much content.
Duolingo has courses in 34 languages for English speakers—excluding languages from works of fiction—are: Arabic, Chinese, Czech, Danish, Dutch, Esperanto, French, German, Greek, Hawaiian, Hebrew, Hindi, Hungarian (in beta), Indonesian, Irish, Italian, Japanese, Korean, Latin, Navajo (in beta), Norwegian, Polish, Portuguese (Brazilian), Romanian, Russian, Scottish Gaelic, Spanish, Swahili, Swedish, Turkish, Ukrainian, Vietnamese, and Welsh. A Finnish course is in development.