Esperanto is the world's most widely spoken constructed international auxiliary language. Created by L. L. Zamenhof in 1887, it is intended to be a universal second language for international communication, or "the international language". Zamenhof first described the language in Dr. Esperanto's International Language, which he published under the pseudonym Doktoro Esperanto. Early adopters of the language liked the name Esperanto and soon used it to describe his language. The word esperanto translates into English as "one who hopes".

Duolingo, Inc. is an American educational technology company that produces learning apps and provides language certification. Duolingo offers courses on music, math, and 43 languages, ranging from English, French, and Spanish to less commonly studied languages such as Welsh and Irish. Other services include the Duolingo English Test, an online certification program, and Duolingo ABC, a literacy app for children. The company uses a freemium model, with its optional premium service, Super Duolingo, being ad-free and offering more features.