Can anyone help identify the earliest piece of fiction to show a future where Esperanto is widespread?
Here are the two candidates I can propose up front.
"Homo Sol", written Jan. 1, 1940, short story by Isaac Asimov. An extraterrestrial character is quoted saying "I delivered the invitation before their parliament in their own language–a simple one they call Esperanto." There were some things this character didn't know about Earth, but I'm sure he did enough research to know the language, so I'm confident this counts.
"The War in the Air", written in 1908, novel by H.G. Wells. It has this quote "Bert came to houses of the same detached, unwalled, wooden type, but adorned now with enamelled advertisements partly in English and partly in Esperanto." The novel does not say whether Esperanto ads are common in the future, or whether it's just the habits of one eccentric shopkeeper. Esperanto is certainly not overwhelmingly popular, because this novel has a lot of language barriers between people speaking English, French, German, and Czech. I don't know whether this counts or not without looking more into what the author intended.
Things that are out of scope for this question (but interesting enough for me to be curious about them)
- Non-fiction predictions
- Using Volapük or a fictional auxiliary language
- Using Esperanto as a stand-in for another language, like in the comic book series "Saga"
- Works not set in the author's future, like the movie "The Great Dictator", which features fictitious countries
- Works where Esperanto is no more widespread than it was in the author's time
Related questions on Esperanto Stack Exchange: Use of Esperanto in Science Fiction, which asks for a list of science fiction works featuring Esperanto, without asking for earliest, and without specifying that Esperanto be widespread in the future.
Ĉu estas iu ajn sciencfikcia filmo en kiu troviĝas Esperanto?, which is like the above, but it's only about films.