At the beginning of the last century, the belief that one day a universal language could replace all others, and put an end to national rivalries and animosity, was by no means considered a fringe one. And there was little doubt that if such a scheme were to triumph, that language would be Esperanto—the invention of a Jewish ophthalmologist from Russian Poland named Leyzer (Ludwik) Zamenhof. Unlike many other Jewish champions of universalism, Zamenhof never turned his back on his people and their particularistic concerns. James R. Russell tells his story:
Sympathetic in his youth to Zionism, in 1882 Leyzer opened a local Warsaw chapter of Ḥov’vey Tsiyyon (“Lovers of Zion”) in the wake of the pogroms that swept the Russian empire. He even met his future wife, Klara Silbernik, at a clandestine meeting of the group. Though in his later years Zamenhof ceased to champion Zionism actively, he never actively opposed it.
Esperanto emerged in the conditions of the Jewish Diaspora, addressed its concerns, was suffused with its hopes, and was shaped by its linguistic environment. It attracted a disproportionately large number of Jewish adherents and its enemies attacked it as a Jewish language. In many ways, it was and continues to be.
[F]or the first two decades or so of the existence of Esperanto, about 90 percent of the movement’s subscribers and supporters were subjects of the Russian Empire. A majority of those were Jews as well, and though statistics are hard to come by, it might be fair to estimate that perhaps as much as a quarter of the present Esperantist community is Jewish.
The fate of the international language under fascism was grim. In Mein Kampf, Hitler had condemned Esperanto as part of the Jewish world conspiracy, and [his] regime outlawed the international language in Germany. To the east, the situation was little better: Esperanto had flourished in the young Soviet Union, where it was seen as a means of getting the workers of the world to unite. But there, too, the Stalin regime, suspicious of its cosmopolitanism and the contacts it facilitated between Soviet citizens and foreigners, banned it, and executed numerous Esperantists.
Read more on Librarians: https://blog.nli.org.il/en/lbh-esperanto/