WARSAW -- It all started last Monday in this city's massive Palace of Culture, when a Swede, a Pole, a Burundian and an Argentine met to swap ideas for replacing English with Esperanto as the international language of diplomacy, commerce and science.

By midweek, the centennial celebration of the Esperanto language had attracted the biggest and perhaps most motley crowd in 72 years of Esperanto annual international meetings.

With the lofty goal of improving international understanding, the festival, like the language, drew a wide range of enthusiasts. In all, there were 6,000 participants from 64 countries.

Not that it was needed, but the conference's Polish hosts threw in a touch of levity with a Thursday night ball. Breaking into Esperanto versions of such tunes as Stevie Wonder's "I Just Called to Say I Love You," a Polish rock band turned the palace into a makeshift disco and the Esperantists into jig dancers all -- Japanese in kimonos, Bavarians in lederhosen and Americans in Brooks Brothers suits.

But it was on a somber note that Esperanto's American leader brought the week to an end. In a closing ceremony Saturday, Humphrey Tonkin exhorted the gathered to spread the word -- noun, verb, adjective, what-have-you -- for another century. "A movement must move," he told a packed crowd of Esperantists. "I believe that Esperanto will conquer the world."

Esperanto was created in 1887 by Polish oculist L.L. Zamenhof. His concept: A language simple in vocabulary and grammar and politically neutral. Within everyone's intellectual grasp, it would eventually be recognized as the world's second language.

So far, the Universal Esperanto Association has 40,000 members, according to Tonkin, who is president of both the association and of the State University of New York at Potsdam. But he and other specialists here estimate that Esperanto speakers range anywhere from 1 million to 15 million.

Esperanto's easy grammatical structure is probably its biggest attraction. Zamenhof introduced 16 basic rules of grammar, with no exceptions. Verb endings, for instance, are the same for the first, second and third person singular and plural. "You can learn the rules in a matter of minutes," said David Featherstone, a 63-year-old retired London schoolteacher. Others said they gained fluency in three months to a year.

Another attraction is that Esperanto is chock-full of words familiar to students from both West and East. Zamenhof took the first 1,000 root words from his wide repertoire of languages, which included Russian, Polish, Hebrew and Yiddish, according to Esperanto folklore. Over time, the root words have increased to 15,000, Tonkin said, including many taken from other tongues.

As a result, hungry Poles can order a "Kolboso" in Esperanto, as many participants at the konferenco did. American speakers can write a letter to their "Senatoro," or ask for a "Divorco."

One of the most unusual things about Esperanto is its wide though uneven range of appeal. Few Americans and French speak it, according to Tonkin. But it appears to have its largest following in China and a sizable one in the Soviet Union as well, he said.

A language without a country, its spread over the course of the past century has been haphazard at best. Featherstone started learning it eight years ago when a friend willed him a New Testament Bible written in Esperanto. Others starting learning it while bedridden in hospitals or through close friends or relatives who spoke it. Some Esperanto speakers stress that it is a literary language. With all nouns ending in "o," it lends itself well to poetic rhymes, said Julius Balbin, a New York linguist who writes Esperanto poetry.

Polish actress and translator Kalina Plenkiewicz went even further, saying certain literary works gain something when translated into Esperanto. Citing translations of Bertolt Brecht's "Threepenny Opera" and certain Polish classics, she said, "I don't want to offend the original authors, but the Esperanto translations are better."

For Los Angeles computer expert John Tuscini, on the other hand, the attraction is that its "no exceptions" grammar structure makes it conducive to computer programming. "This is going to be the kernel of future computer languages," he said. "It's error free."

Cleansed of the nationalistic baggage attached to such languages as English, French and German, Esperanto attracts a large following in Sweden, Austria and other neutral countries. But some argue that the more prominent the language becomes, the more it loses its charm for neutralists.

"People here try to make too much of a personality cult of the language," complained Niklaus Wiess, a Swiss member of the "same-sex-loving male and female Esperantists," or gays.

"I abandoned my native German to try to escape that."