Saluton! I actually saw your question about this on /r/Esperanto the other day as well; perhaps unfortunately, I draw a fair bit from one of the sources brought up in that thread, so this might be more of a preview to things you’ll soon discover than anything else. (I’ll also note upfront that I’m coming at this from the perspective of someone familiar with Esperanto history, not Soviet history). I hope it’s useful, regardless.

For those who are unfamiliar with Esperanto, though, here’s a brief overview. Esperanto is a language created to be a lingua franca for the world, a language easy to learn so that people who don’t share a native tongue could have a common secondary language to communicate with, and as such, there wouldn’t be linguistic barriers between people. It was invented in the late 19th century by Ludwig Zamenhof, a Jewish ophthalmologist who lived in Białystok, Poland, while it was still part of the Russian empire. Because Esperanto was rooted in ideas of international harmony and brotherhood, it was very popular with left-leaning people, who used it to help spread socialist and communist ideas. However, Esperanto as a movement had no political affiliation: the Declaration at the first Esperanto Congress at Boulogne-sur-Mer in 1905 said that while the purpose of Esperantism is to spread the language of Esperanto, that “All other ideals or hopes tied with Esperantism by any Esperantist is his or her purely private affair, for which Esperantism is not responsible.”

In fact, while Esperanto was used to spread leftist ideas, Esperantists have ranged the spectrum of political ideology. There were notable American Esperantists who hated how some people were tying Esperanto to communism and hurting the language’s reputation, and there were also in fact Nazi Esperantists who, seeing how Esperantism was repressed in Germany, sought to strip Esperanto from its Jewish origins in order to protect the language. This… didn’t work out too well.

But I digress: by and large, Esperanto was viewed as a language for communists (which made communism-hating countries not like it too much), and the reputation was fairly deserved, considering they did comprise a significant portion of the Esperanto community. During the Revolution, Russian Esperantists were eager to use the language to help “liberate their oppressed comrades in the West” (Lins 160), and Esperanto groups in Russia increased, reaching around 100 in 1919. Meanwhile in Paris, Eugène “Lanti” Adam founded the Sennacieca Asocio Tutmonda (Worldwide Anational Association) in 1921 to broadly promote workers’ rights, anti-capitalism, and other such left-wing beliefs; it reached 6500 members in 1929. He hoped to use SAT to unify social democrats, communists, and anarchists through Esperanto, in order to lead working classes of all nations to revolution and bring about a new social order for the worldwide proletariat. (Lanti, incidentally, was the uncle of a young George Orwell, who accompanied him at some Esperantist outings. This likely led to some Esperanto influence when Orwell developed the language Newspeak for his novel Nineteen Eighty Four, as there are some similarities between the languages). For years, though, Lanti and the SAT faced hostility from Ernest Drezen, who in the 20s became president of the Soviet Esperanto Union. Although the SEU attempted to get a long with SAT, Drezen, also a communist, felt that Lanti’s dream coalition members would be “more dangerous enemies for the revolutionary movement than the openly bourgeois” (qtd. in Schor 149), and instead focused on promoting Esperantism among Soviet workers, and use that influence to spread Esperanto and get more institutional support for the movement in Europe.

As you note in your question, Esperanto in the Soviet Union starts out fairly alright. Thanks to the work of people like Drezen, Esperanto was taught in Soviet factories, textbook sales were popular, it became an optional class in Soviet schools, and Esperanto theatre was supported by the state—more so than, say, Jewish or Roma theatre (or education, or people in general). Drezen encouraged SEU members to correspond with foreign workers: as Ulrich Lins notes, “in 1925–26 around 2000 letters in Esperanto were mailed in a period of eight months from the cities of Minsk and Smolensk alone” (180), and this contact brought workers from Britain, Germany, and several other western nations to visit the Soviet Union. Thanks to aligning viewpoints, and despite some initial suspicions, the Soviets treated Esperantists generally decently in the 20s.

But the honeymoon never lasts forever, and some discontent among Esperanto letter-writers led to Soviet suspicions that the SEU was trying to bring in bourgeois ideas toward the end of the decade. As it turned out, some Esperantists weren’t a fan of living in Soviet Russia, and made that clear to the foreigners they wrote with. Esperanto also clashed with Stalin’s own post-1929 plans for a world language. Tensions escalated both among disparate Esperanto groups in Russia, and between them and Soviet leadership, and long story short, the Soviets stopped trusting the Esperantists, or any other alternate ideology. By the end of the 30s, Esperanto clubs had been shut down, magazines were cancelled/censored, and of course, the purge happened, Drezen included among the many victims. Esther Schor recounts for us (190):

The onset of the Great Purge in 1936 found the SEU keeping a low profile, publishing theories of language pedagogy and advertising its usefulness to foreign-language instructors. But once the purge began in earnest, Esperantists were persecuted as individuals with suspicious ties to those in other countries. One by one, the luminaries of the Soviet Esperanto movement disappeared from view. Rank-and-file members were also arrested, interned in labor camps, and killed. Precise figures are hard to come by; one soviet Esperantist estimated that upwards of thirty thousand samideanoj [fellow Esperantists] were arrested and several thousand died.

Over the next few decades, public Esperanto activity in and around the Soviet Union was just about impossible, though the level of suppression varied from place to place. Sporadically, we see several Esperanto groups in some parts of Eastern Europe have brief moments of hope, only to have it shut down and not return for many many years. Soviet leaders continued to view Esperanto as a way for foreigners to infiltrate their ideas into the USSR, rather than a way to spread support beyond its borders. Arrests continued into the 1950s in some countries, though fewer Esperantists were sent to labor camps. Public meetings had to be approved by local authorities, but many feared ostracization or some other consequence, so they rarely actually held them.

Following Stalin’s death in 1953, the Soviet Union sought to rejoin the world landscape, and alongside that renewed its sympathy for Esperanto. Those punished by the purge were exonerated, Esperanto institutions grew, and Esperantists still living in the Soviet Union got to find out more about what was happening with Esperanto elsewhere in the world. Reaccepting Esperanto into society was a long and arduous task, though, and while it varied from country to country, it was rarely hunky-dory. But, y’know, it did get better.

Further Reading

My main source, as you may guess, is Ulrich Lins’s Dangerous Language. I primarily cited the first volume, Esperanto Under Hitler and Stalin, but I also consulted the second volume as well, Esperanto and the Decline of Stalin.

For more general history, Asya Pereltsvaig gives a summary/literature review of Esperanto history in her article “State-of-the-Art: Esperanto History”, which features some discussion about Esperanto and the Soviet Union somewhere in the middle. Going more in-depth, I also recommend Bridge of Words by Esther Schor, which is a more comprehensive look at Esperanto’s history. There isn’t a single chapter that is focused on Esperanto and the USSR, but she puts together bunches of the story through various narratives throughout the book. These both give an accessible overview of the topic, though both draw a lot on Lins’s work.