WHEN THE LEAGUE OF NATIONS first convened in 1920, a universal language was on the agenda. Delegates from a dozen countries, including Brazil, China, Haiti, and India, declared their hope “that children of all nations from now on would know at least two languages, their own mother tongue and an easy means of international communication.” The most likely candidate for such an “international auxiliary language” was Esperanto, launched only 33 years earlier by Polish-Jewish ophthalmologist Ludwik Zamenhof. At its height, the lingvo internacia was embraced by working-class Jews, French intellectuals, East Asian leftists, Baha’i believers, Shinto sectarians, and Brazilian spiritists. French was in decline, English not yet completely ascendant, and the delegates believed that Esperanto could fill the vacuum, starting in schools and telegraph offices and spreading inexorably as both the medium and the message of international cooperation and world peace.

It’s easy to see why the League was so enamored. Modern, efficient, progressive, Esperanto embodied a certain spirit of early 20th-century internationalism. Channeling what he called “the spirit of European languages,” Zamenhof had forged a hybrid of Romance, Germanic, and Slavic elements, streamlined for maximum transparency (no irregular verbs!) but distinctive and ingenious when it came to word formation. Chains of prefixes and suffixes work virtuoso wonders in Esperanto, with –eg- making anything bigger, -et- making anything smaller, and mal- turning anything into its opposite. More thoroughly and elegantly than the English suffix –ly, Esperanto’s –e transforms any noun into an adverb; Kiel vivi vegane is an Esperanto pamphlet whose title translates to “How to Live Veganly.” There is no lack of idioms, slang, or linguistic color.

Though the League of Nations eventually sent the Esperantists packing after three years of debate — official French opposition was apparently decisive — Zamenhof’s followers would ultimately outlast the League itself. While it never achieved the fina venko (final victory) projected by its more devoted acolytes, Esperanto is today “a living language with a worldwide community,” reports Esther Schor in her fascinating new history Bridge of Words: Esperanto and the Dream of a Universal Language. The language has survived derision, repression, and the onslaught of global English, and it still has an estimated million-plus speakers in a hundred-odd countries. Probably in decline, numerically speaking, Esperanto is still flourishing online, with 230,000 Wikipedia articles and counting. There is even a small world of denaskuloj, native Esperanto speakers from birth.

No other language constructed largely by a single individual has ever matched Esperanto in popularity, or seems likely to any time soon. Why Esperanto — and not Lingua Ignota, Volapük, Interlingua, or any of a thousand other constructed languages? As “conlanger” David Peterson, who invented Dothraki for Game of Thrones, points out in The Art of Language Invention, every consciously created language bears the imprint of its era. Medieval languages for addressing God, like the mystic polymath Hildegard von Bingen’s Lingua Ignota, gave way to “philosophical languages” in the 17th and 18th centuries, which sought to encode the structure of all knowledge. (Imagine the Dewey Decimal System as a spoken tongue.) Today the hobbyists of the Language Creation Society, inspired by sci-fi and fantasy but ever more informed about Earth’s linguistic diversity, share “artlangs” online.

Esperanto is a product of the late 19th and early 20th centuries, an era that saw a rage for constructed languages linked to some kind of cause or hope for social reform. The Eastern European Jewish milieu in which Zamenhof was raised was uniquely hospitable to language planning; he was born just a year after and a few hundred miles away from Eliezer Ben-Yehuda, the founder of modern Hebrew. Before the runaway success of Dr. Esperanto’s International Language, his 1887 opus now universally known as Unua Libro (First Book), Zamenhof was trying his hand at Zionist activism and Yiddish language reform. For a meager living, he made eyeglasses for the Warsaw poor.

The Unua Libro humbly pitched Esperanto as “an official and commercial dialect,” an easy cipher designed to save its speakers time and money. A century before Linux, the language was avowedly open-source: there was a one-year open comment period during which anyone could vote on proposed changes, and Zamenhof declared that “the future of the international language is no longer more in my hands than in the hands of any other friend of this sacred idea.” The timing was fortunate, too: Esperanto launched right after the collapse of Volapük, invented in 1879 on divine inspiration by the German Catholic priest Martin Schleyer. Saddled with endless, intricate verb endings, Volapük apparently never transcended its user base of “male, educated, German-speaking Catholics,” according to Schor.

Esperanto’s center of gravity moved early on to France, but Zamenhof retained at least nominal leadership until his death in 1917, issuing reforms and fighting off a schism with the offshoot language Ido. By then, the Universal Esperanto Association (UEA) and the Akademio de Esperanto were leading the way, managing a perennial rift between those who proclaimed the language’s political and ideological neutrality and those who linked it to the interna ideo, “an undefined feeling or hope,” in Zamenhof’s words, that would eventually lead to “a special and completely defined political-religious program.” Core tenets of Zamenhof’s little-heeded program of Hillelism, later called Homaranismo (“Humanity-ism”), were belief in a higher power, individual conscience, and moral reciprocity (Rabbi Hillel’s “Golden Rule”). For other, more secular-minded Esperantists, the “internal idea” was socialism, pacifism, or anti-nationalism.

Esperanto has always been weak in the United States, though not for lack of trying by its pioneer, the Irish immigrant Richard Geoghegan, who eloped with a woman from Martinique and became a stenographer in Alaska, where in his spare time he wrote a dictionary and grammar of the Aleut language. Carrying the lingvo in the other direction was Vasili Eroshenko, a Ukrainian violinist and anarchist, blind from the age of four, who propagandized for Esperanto in Tokyo and Beijing, where he worked as a masseur and lived with the famous Chinese writer Lu Xun. Deported on multiple occasions, Eroshenko eventually turned up in the Soviet Union, where he taught Communist operatives and studied the Chukchi of Siberia, publishing the results in an Esperanto Braille journal. (One advantage of an intentional linguistic community is that the ratio of characters to speakers tends to be high.)

Eroshenko survived, but thousands of other Esperantists were purged under Stalin: bitter testimony to the strength of the language on the left (though there was also, briefly, a handful of Esperantists aligned with the Nazis). Officially, Esperanto remained a language of non-alignment, but the Cold War split the granda rondo familia in two. For many on the Soviet side, the language became a bridge to the wider world — quite literally for denaskulo investment whiz George Soros, who defected to the West while attending a 1947 Esperanto congress in Bern. Fidel Castro hosted a garden party for Esperantists at the Havana Congress of 1990. In recent decades, estimates Schor, China may have funded Esperanto more generously than any other nation, publishing the glossy periodical El Popola Ĉinio (From the People’s China). UEA membership plummeted after 1989, once hard-up “Eastern Europeans had neither the motivation nor the leisure to pursue Esperanto,” making way for the full-scale expansion of English across the former Soviet Union.

Today Esperanto is as much a movable feast as a movement, a cheerful diaspora that lives on at characterful classes and congresses where diehards for the interna ideo mingle with fearsome polyglots and hardcore language nerds. A language spoken by a mobile subculture of (mostly) middle-class Westerners may have a better chance at survival than most of the world’s natural languages, half of which are endangered and have fewer than 10,000 speakers. The greatest danger Esperanto faces is that a language born out of a hope for universal understanding could end up as just another hobby, cultivated in convention halls that next week will be filled with memory junkies, chess fanatics, or Trekkies.

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For a would-be international language, the paradoxical strength of Esperanto lies in its particular identity and idealism. Born in an age of nationalisms, it sometimes seems like a language in search of a country. There’s no army, but Esperanto has many of the other trappings of a nation-state: a flag, an anthem, a literature with its own rigorous poetics, and even places of pilgrimage like Zamenhof’s Bialystok and an Esperanto-speaking farm-school in the Brazilian Amazon. In 1996, the UEA pledged itself to support seven core objectives: democracy, global education, effective education, multilingualism, language rights, language diversity, and human emancipation. Its grammar may have a geeky appeal, but plainly humanism, internationalism, and love of language are Esperanto’s bedrock.

In what may be the most dramatic reinvention of all, Esperanto’s granda rondo familia (great family circle) is returning in a roundabout way to the Jewish roots it once fought to transcend, notably at the first-ever World Esperanto Congress held in Boulogne-sur-Mer, France, in 1905. As the dream of becoming the world’s language becomes increasingly remote, many Esperantists are embracing the status of “self-elected diasporic minority,” proclaiming themselves speakers of a language just as worthy and particular as any other. Bridge of Words is a guide to this modern-day diaspora, from a participant-observer attuned to the Jewish dialectic of universalism and particularism. (Schor is a scholar of Jewish literature at Princeton and the author of a biography of the American poet, and proto-Zionist, Emma Lazarus.) “I’ll confess that at Esperanto gatherings, I sometimes feel that I’m among meta-Jews,” she writes. “After all, Esperanto was invented by a Jew who renounced peoplehood, but couldn’t imagine a world without it.”

In an age of mass linguistic extinction, Esperanto is an unlikely point of light, a rare example of a language community that relies less on transmission from parents to children than on the passion of self-selecting adults concerned explicitly with communication and justice. If English is an expanding empire, Esperanto is a quirky and fractious co-op, still hanging on a century after its creation. For those who keep faith with Zamenhof’s dream, globalization without empire remains an animating possibility. “Nostalgia for world culture,” as the poet Osip Mandelstam called it, is still an idea with a future.

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Ross Perlin is a writer and linguist based in Brooklyn.