Short answers, pulled from the story.
Esperanto was created by L. L. Zamenhof, a Jewish ophthalmologist from Bialystok, then part of the Russian Empire. He published the first grammar booklet, Unua Libro, on the 26th of July 1887, under the pseudonym Doktoro Esperanto.
Esperanto translates into English as "one who hopes." It was the pseudonym Zamenhof used when publishing the language's first book, and early adopters adopted it as the name of the language itself.
Estimates range widely. Sidney Culbert's research over twenty years placed fluent speakers at between one and two million. A 2017 doctoral study by Svend Nielsen estimated around 63,000 active speakers, while Jouko Lindstedt's 1996 model suggested roughly 10,000 fluent speakers and up to one million passive understanders. Approximately 2,000 people have Esperanto as a native language.
In Nazi Germany, Esperanto was banned in 1935 partly because its creator Zamenhof was Jewish, and because the language's internationalist nature was viewed as "Bolshevist." Adolf Hitler mentioned it in Mein Kampf as a potential tool of an international Jewish conspiracy. In the Soviet Union, Stalin reversed earlier government support during the Great Purge of 1937, and many Esperanto speakers were executed, exiled, or sent to Gulag labor camps; repression lasted until the end of the Stalin era in 1956.
The most widely used Esperanto flag features a green five-pointed star against a white canton on a green field. It was proposed by Richard Geoghegan in 1887 and approved by delegates at the first Esperanto conference in Boulogne-sur-Mer in 1905. The green star, known as la verda stelo, is also used alone as an emblem to identify speakers to one another.
Duolingo launched a free Esperanto course for English speakers on the 28th of May 2015. By July 2018, the number of registered learners on that course had risen to 1.36 million. Google Translate added Esperanto as its 64th language on the 22nd of February 2012.