The sponsor of a bill to make English the official language of the United States is Sen. Steve Symms, whose home state of Idaho is not exactly a bubbling vat of ethnic tensions. Polyglots, rest assured, are no threat to the character of Boise. But this is an issue that confounds ordinary political calculations.

California, the most robustly heterogeneous state, gave lopsided approval last November to a referendum endorsing English-only ballots. Florida, whose population at last count was fully 9 percent Hispanic, is now the site of a petition drive, led by a group called U.S. English, to make English the official state language. The group says five states, including Illinois, already have taken that step.

Naturalized Americans may regard these as expressions of bigotry and xenophobia. For some people, they probably are. But bad motives and cranks can be found in any movement. This one nonetheless promotes changes that are needed to preserve the unity of a fiercely diverse nation.

The persistence of foreign languages among ethnically distinct peoples is not a new phenomenon, contrary to popular impression. Thirty years ago, New York City had daily newspapers in Italian and German; it still has a Yiddish- language daily. Chicago has dailies not only in Spanish but in Polish and Lithuanian. Yet Jews, Poles and Lithuanians have assimilated anyway.

One reason is the lack of an alternative. They, not native-born Americans, have always been expected to adapt, or else pay the price. The difference today is the government`s systematic effort to accommodate some people who don`t speak English--in bilingual education, ballots, licensing exams, contracts, and so on.

The Florida initiative is supposed to stop the use of Spanish in most state publications, which tends to discourage immigrants from a brisk transition to English. The proposal is only a small blow against the persistence of insular ethnic communities--private businesses would still be free, as they ought to be, to use whatever language they want. There will be no language police.

But the measure is worthwhile for whatever concrete progress it would bring, as well as for its symbolic value. It recognizes that the durability of the American nation owes a critical debt to the unifying power of a common language. If Louisianans had clung to French, Californians to Spanish and Minnesotans to Swedish, American history would be a different story--or, more likely, stories.

Ethnic diversity enriches the life of a community. But it can survive in a nation only if it is offset by stronger homogenizing factors. The danger is that a large ethnic enclave, encouraged to seal itself off from the greater community, will grow culturally and politically estranged.

Just as a shared tongue unifies, different ones can inexorably divide. The separatist movement in Quebec is only the latest and closest illustration. In this century, no force has been more destructive of established political orders than ethnic nationalism. By contrast, even a nation with 265 different kinds of cheese, as Charles de Gaulle described France, can be preserved by a universal attachment to the French tongue.

For some Americans, the answer is more drastic--simply shut out foreigners. But immigrants who are willing to assimilate have always been an incomparable asset to this country. Designating English as the official language would underscore the importance of rapid adaptation.

What the Hispanic groups who oppose the change should consider is that a liberal immigration policy depends on the certainty of assimilation. The latent popular hostility toward new arrivals is largely a product of fears that they will resist integrating themselves into American society.

The Statue of Liberty has always stood as an invitation to people around the world to become Americans. Its inscription--''Give me your tired, your poor, your huddled masses yearning to breathe free''--is, not by accident, in English only.

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