MUSIC IS the international language, yes. But there are a lot of dialects.

When Mstislav Rostropovich came to Washington five years ago, he spoke hardly any English at all, just enough to charm the pants off everyone. And talking to the orchestra he could get along pretty well with Italian, since the technical terms of music are nearly all in that language.

"Pianissimo," he would say, and the orchestra would nod and play pianissimo. "Pizzicato," he would say, and the orchestra would dutifully pluck at the strings.

But what happened when he wanted the lights turned up?

What happened was that he got a permanent interpreter.

She is Nadia Efremov, and she is still on the job, though the maestro's English has improved enormously.

"He does puns now," she says. "He's quite at home in the language. He's got a fabulous memory, a wonderful ear. He picks up the most amazing things."

True, he can't read or write the language to speak of, and his syntax is very special, and if he doesn't get a word he can't ask you to spell it because he is still operating with the Cyrillic alphabet.

Efremov has a Cyrillic attachment for her typewriter at the Kennedy Center, where she has an office as Rostropovich's official secretary.

A native of Smolensk, she fled Russia as a small girl when the Germans invaded, wandered with her family across Germany and finally reached the American zone in 1945. The family came to America the next year, and she learned her English in school.

"I was free-lancing for America magazine," she says, "when my boss asked me if I wanted this part-time job with the National Symphony. Now I'm part-time at home."

Rostropovich has a way of filling up the lives of those around him. When he is around, her working day runs about 20 hours. Her husband, a Russian born in Yugoslavia and an executive at the Federal Communications Commission, has learned to cook and help take care of the two children.

"I go on the tours, of course," she says. "I go to the receptions and all. I'm needed for the toasts and speeches. He doesn't talk the way the rest of the world talks. He has a very convoluted brain, and he starts these elaborate sentences, he starts off in left field and gradually gets around to the point. Then he socks it to 'em. It's not easy to translate sometimes."

But the maestro, an exquisitely sensitive man, can tell when she is having trouble and slips in an extra word or two for clarity if she hesitates.

One problem was his Soviet slang. She didn't know a word of it. And her American slang was a new world to him, especially the euphemisms she had to use for his rather earthy language.

Because Rostropovich's wife is off, usually in Europe, pursuing her own career much of the time -- though they are most happily married after 26 years, she adds -- Efremov is expected to deal with the artistic chaos of a much-traveled conductor's life.

"I'm the only one who knows his sock size and shirt size. Once I got a call in Washington from Harrod's in London, saying that he was trying to order a certain kind of shirt and had told them I'd know what he meant. Sometimes he'll call from Europe and forget it's 3 a.m. here. But you get used to that."

He is endlessly patient, she says, with mistakes. And kind.

"When my daughter first got her braces and was terribly self-conscious, he took her aside after a concert and talked to her for 15 minutes or so. Now, that was a very caring thing to do. He's a very caring man."

Being Rostropovich's interpreter has changed her life, all right. It's given her some clout with her children. She used to have to nag them about keeping up their Russian, "but now they see me going to the White House and visiting the pope and things like that, and they figure it must be good for something!"