Who created the constructed language Esperanto, and when?
He wanted a neutral second language for everyone.
Esperanto was devised by Ludwik Lazarus Zamenhof, a Polish-Jewish ophthalmologist from Bialystok, then part of the Russian Empire. He published the first textbook in 1887 under the pseudonym 'Doktoro Esperanto', meaning 'one who hopes', and the language eventually took that name. His aim was a neutral second language that would ease communication between peoples who were often in conflict in his multilingual home region.
The language was built to be regular. Its grammar is based on a small set of rules without exceptions: nouns end in -o, adjectives in -a, adverbs in -e, and word families are built with prefixes and suffixes. Much of the vocabulary comes from Romance languages, with contributions from Germanic and Slavic languages. Learners generally report that it is quick to pick up compared with natural languages, though quantifying that is difficult.
Its history was not smooth. The first World Esperanto Congress was held in Boulogne-sur-Mer, France, in 1905. Smithsonian Magazine notes that the language was suppressed under both Hitler and Stalin because of its internationalist associations. Estimates of the number of speakers vary widely, from tens of thousands to a couple of million depending on how fluency is defined, and a small number of people have learned it as a first language from birth.