Zamenhof spoke of his native language as being his father's Russian (although some people have suggested that Belarusian would be more accurate), but he spoke his mother's Yiddish as his main family language. It is interesting that once Ludwig’s own children were born, Polish became the home language of his children. German of course was not difficult for him to pick up, since Yiddish is in any case 70% German. Later he learned French, Latin, Greek, Hebrew and English well, and a couple of other European languages less well.
Just to make the question of languages personal, my own grandfather spoke Yiddish as his home language and Russian as his day-to-day school and work language. He too spoke excellent German and Polish by the time he was a young adult, but he had to learn French, Hebrew and English the hard way – via language classes and books. Greek and Latin would have been out of the question for Jewish males in my grandparents' part of Russia (now the Ukraine). But one language my grandfather had over Zamenhof was Italian. Australia was filling up with Italian migrants in the 1950s and 60s, and no professional could afford not to learn Italian.
Anyhow in addition to the Yiddish-speaking Jewish majority, the population of Białystok had lots of Poles, Germans and Belarusians. There were endless struggles between the subgroups of this part of the Russian Empire, which I would put down to religious and political differences, but Zamenhof thought otherwise. He believed that the main reason for hatred lay in mutual misunderstanding, caused by the lack of a common language that everyone could understand, regardless of their different ethnic backgrounds. Was he naïve in thinking that a shared language could play the role of a neutral communication tool between peoples?
After years of working on this new international language, the first book of Esperanto grammar (The Unua Libro) was published in Warsaw in July 1887, in Russian. Zamenhof was a productive thinker and writer; he also spent years translating literature into Esperanto.
The linguists say that as a modern language designed from the ground up, Esperanto is not genealogically related to any specific language. The sound, grammar, vocabulary and semantics are largely based on the western Indo-European languages. The semantics are essentially Slavic, while the vocabulary derives primarily from the Romance languages, with some words coming from German. Pragmatics and other aspects of the language not specified by Zamenhof's original documents were influenced by the native languages of early speakers, as you might expect: Russians, Poles, Germans and French-speakers.
The number of speakers grew rapidly over the next few decades, at first largely in the Russian Empire and Eastern Europe, then in Western Europe and abroad. Journals appeared. Finally in 1905, the first world congress of Esperanto speakers was held in France. Since then world congresses have been held in different countries every year, except during both World Wars.
Providentia blog is excellent on the history of Esperanto in the era of rising Fascism in Central and Eastern Europe. "In his work, Mein Kampf, Adolf Hitler specifically mentioned Esperanto as a tool of international Jewish Conspiracy and the language that they would use once they dominated the world. It probably didn't help that Esperanto use became popular in Eastern Europe and the Soviet Union and, as such, became strongly associated with Bolshevik movements".
Almost no-one in the entire universe speaks Esperanto with their children at home, instead of a more usual native language. Probably some 10 million people have studied it at school or university, but only one university in the world gives its lectures and tutorials in Esperanto: Akademio Internacia de la Sciencoj, San Marino.
For Zamenhof and other Esperanto fans, this language was far from being merely a communication tool. Rather I think they saw it as a way of promoting the peaceful coexistence of different people and cultures. But this was the very goal that made nervous national leaders turn against Esperanto. Nazi Germany, for example, prohibited Esperanto and gaoled its followers because a] Zamenhof was Jewish, b] Esperanto was internationalist, not nationalist and c] most followers were pro-peace and anti-war. Joseph Stalin believed that Esperanto was the language of spies; his henchmen killed several thousand Esperanto-speakers in 1937, calling them Enemies of the People.
Zamenhof had not been an avid supporter of a Jewish homeland because he was "profoundly convinced that every nationalism offers humanity only the greatest unhappiness". But all this international understanding didn't help the Zamenhof family. Ludwig died from a heart attack in 1917, only 3 years into the War to End All Wars. And as Providentia blog explained, Ludwig's three children were all shot by the Nazis during the next war - two of them had been brilliant doctors and one was a noted educator. Had they escaped to a Jewish homeland by 1939, the family might not have been exterminated.
