What is the etymological origin of the language learning app name 'Duolingo'?

In the world of language learning and linguistics products, brand names often have fascinating origins that connect to their purpose or identity. Some draw from classical languages, others from linguistic concepts, and some have surprising etymological roots. This trivia challenges you to identify the true origin story behind a well-known language learning brand's name. Test your knowledge of linguistic product branding!

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From Latin 'duo' (two) and Spanish 'lingo' (language), representing bilingual learning
From Latin 'duo' (two) and English slang 'lingo' (language), representing the dual aspects of reading and speaking
From Italian 'duolo' (pain) and English 'lingo' (language), humorously referring to the challenges of language learning
From the founders' names Duncan and Lingo, who created the initial algorithm