By “human” language, they mean “English” language. This is a study of English only, using these sources:

> The NRC VAD lexicon … English Fiction (1900 to 2019) from the Google Books project … Jane Austen’s six novels … The majority of Arthur Conan Doyle’s Sherlock Holmes stories … The New York Times (1987 to 2007) … Wikipedia (English language, March 2019 snapshot) … RadioTalk (transcriptions of talk radio broadcasts in the United States, October 2018 to March 2019); and Twitter (approximately 10% of all tweets identified as English in 2020).