Longfellow limited his travels to Europe, where - except for indigenous folk music in rural areas of Sweden and France - the only music he heard was in the mainstream o f Western tradition. He might not have been so quick to generalize had he heard the classical music of, say, Persia, Japan, Korea, India or Indonesia.

The fact is that if music is a language at all, it is certainly not a universal one. It's true that the principles of acoustics know no national boundaries, and thus one encounters world-wide an emphasis on the octave and a predilection for five-note scales based on the overtone series. But these are matters of science, not art. Once a listener moves beyond the borders of his native culture, he can discover countless kinds of music that are strikingly different from his own. Not only is such music foreign in sound; it is foreign in its goals and purposes, in what it ''means'' - or is supposed to mean - to the persons who experience it.

Typically it's only the most superficial elements of an exotic music that need no translation.

Western ears have little trouble focusing in on the twangy tone-colors of Hindustani sitar playing, for example. The colors are fascinating and attractive; as anyone who paid attention to British and American pop music in the psychedelic '60s remembers, they're also very easily imitated. There are few Westerners, however, who can hear past the surface colorations and grasp a North Indian raga's real significance. We can read in a book that certain ragas are supposed to be played only at certain times of the day during certain seasons. But unless we've been thoroughly steeped in Hindu religion and philosophy, an afternoon raga specific to the first weeks of autumn will never ''speak'' to us the way it speaks to an Indian.

It's the same with Javanese gamelan music, a sample of which will be offered to St. Louisans on Saturday evening in a concert - sponsored by the Endangered Arts Foundation - at Washington University's Edison Theatre. We'll be able to take in the dazzling array of metallic percussion sounds and enjoy them to the fullest; most of us, though, won't be able to ''get the message,'' not unless we make an effort to understand what the gamelan is really all about.

The name Gamelan Nyai Panjang Sari applies as much to the collection of gongs and chimes as to the people who on Saturday evening will play on them. It and they are based in Chicago, a city that first made the acquaintance of gamelan music during the Columbian Exposition of 1893. The instruments Javanese performers brought over almost a century ago were acquired by the Field Museum of Natural History, but they sat in the storage room until a visiting curator discovered them in 1976. Interest was strong, and an amateur ensemble was formed to explore the gamelan's possibilities. In 1981 they organized themselves into a group called Friends of the Gamelan, Inc. But the fact that the museum owned the gamelan meant that access to it was limited. So a few years ago they purchased their own gamelan, custom-made for them by one of Java's most prestigious ironsmiths.

Gamelan Nyai Panjang Sari means ''venerable spirit of gamelan.'' All gamelans are in a way venerable, for it is believed - in modern Java as in ancient times - that deities dwell in the instruments themselves.

According to legend, once upon a time - before the island was populated by humans - the chief god Sang Hjang Batara Guru made a gigantic gong for the purpose of communicating with the other gods. A state of chaos resulted after the gong strokes developed into a fairly complex vocabularly. To make his messages more clear, Sang Hjang made a higher-pitched gong and used the two of them in alternation. That too got complicated, so he fashioned yet a third gong, still higher-pitched. Later, after humans came on the scene, Sang Hjang was reincarnated as the god-king Sri Panduka Maharadja Dewabuddha. He remembered the gong business and invented a musical art form - reserved for sacred occasions - that would appeal to mortals as well as to the gods. Thus the gamelan.

Archeologists tell a less colorful story when they explain that the earliest known Javanese gong - a huge horizontally suspended thing called a ''bronze kettledrum'' - dates from only about the second century B.C. It was a ceremonial instrument used for marking special occasions and sometimes for summoning the members of the community; smaller versions, supported by two men and whacked by a third, were used in military situations. The bronze, interestingly, came not from Java but from the mainland, brought in the form of weapons by would-be invaders who were defeated by the Javanese. Bronze was a precious alloy, for the metals needed to make it did not exist on Java; that the Javanese chose to melt down their enemies' swords and cast them into musical instruments, rather than weapons, says something about their values.

hether invented by gods or men, gongs in Java were regarded as sacred objects right from the start. They're still sacred, which is why the largest gong in a gamelan always hangs higher than the others, why the players always remove their shoes before going on stage, why the players never step over an instrument when they move from one area of the stage to another.

The music, too, is sacred, but only in a metaphoric way. Gamelan music in Java serves many functions: to accompany shadow-puppet plays, to celebrate holidays and religious feasts, to usher in the tenures of newly appointed government officials, to entertain and enlighten listeners at concerts.

Most of the compositions, though, are fairly abstract; they have specific titles that refer to their points of origin, but they also have long generic subtitles that identify their tuning system, their mode and - perhaps most important - their rhythmic structure.

The large-scale rhythmic structure will doubtless prove elusive to most listeners at Saturday's concert; a repeating pattern of as many as 256 beats per cycle, with subdivisions on every 64th count, is something Westerners are not accustomed to perceiving. These extended cycles of counts are closely related to the cycles of days and years that make up the Javanese calendar. The calendar cycles are ongoing; they have no beginnings and no endings, because life in Java perpetually renews itself. The gamelan cycles have beginnings and endings only because of practical necessity; in theory, gamelan cycles go on forever, and the gong strokes that articulate them merely poinpoint moments in eternity.

However foreign they may seem, those concepts are worth thinking about as one listens to the 15 or so members of the Venerable Spirit of Gamelan - about half of whom are Caucasian Americans - perform on Saturday. The music means something, and it means far more than what meets the ear. But what meets the ear should, in itself, prove fascinating, especially for persons who have never before encountered a gamelan.

Much of the information in this article was drawn from ''Music of the Whole Earth,'' a 1977 book by David Reck that abounds not just with technical information about non-Western musics but also suggestions as to how Westerners might approach them.