CANADA'S LARGEST life-insurance company voted last week to move its head office from Montreal to Toronto. That constituted a political statement of incalculable force. The company, Sun Life Assurance, is a powerful symbol to the French-speaking separatists who now control the government of the province of Quebec. To them, it stands not only for wealth and investment, but for the dominance of the province's economy by English-speaking Canadians. One reason for the move is that the company wants to assure its policyholders that it will continue to operate under Canadian law. Another reason is Quebec's new language requirements.

Most big companies in Montreal, and throughout Quebec, use English as the language of top management. That's been a very sore point with the five-sixths of the province that speaks French. It means that the French Canadians who is not bilingual cannot hope to rise far in the business world. The provincial government has now enacted legislation requiring companies to use French all the way up, internally as well as in their relations with the French-speaking community.

Sun Life does business all over Canada, and in the United States and Britain as well. It has evidently decided that it cannot go French. So far Sun Life has moved only a legal designation, not the 1,800 people who are its headquarters staff. But many other corporations and banks have been quietly shifting operations - and jobs - into the English-speaking provinces. How fast? It's hard to say precisely. But there's a clue in the population numbers. Quebec has been losing population for some years. In 1975, net outflow was about 12,000 people. But in 1976, when the separatists came to power, it went to more than 18,000. Last year, it was over 46,000.

Quebec has had for years an unemployment rate higher than the Canadian average. The movement of population off the land into industrial work came later in French Canada, with its strong traditions, than in the rest of the country. But in the past two decades young people have been flooding into the cities faster than the economy can efficiently absorb them. It's an important part of the explanation for the force that the separatist movement has suddenly acquired. But this longstanding disparity in unemployment between Quebec and the rest of Canada seems to be widening. For Canada last year the rate was 8.1 percent, and in Quebec it was 10.3 percent. By the beginning of this year Canadian rate was about the same, at 8.3 percent, but the Quebec rate was up to 11.4 percent.

The politics of separatism and language is expensive, in economic terms. For Americans, who take their national unity for granted, it's a reminder of the price that serious political instability imposes not only on society as a whole, but on individual citizens as jobs evaporate, property values decline and opportunities vanish.

American politics, in the present period, goes forward with a good deal of clatter and excitement. But the debate rarely goes very deep. In Canada, the national style of politics tends to be quiet and sedate. But the issues there are the ones that lie at the center of any political system: Why hold a country together? What do these 23 million people have, together, that they would not have if they separated? Can communities with different languages and cultures live together under one flag? The federal government is doggedly defending the principle that people ought to be able to use whichever language they want. The separatists say that Quebec is French, and business there can either go French or leave. Another one has now left.