While Americans are watching the Reagan administration's expriments in budget cutting and tax breaks for the affluent as the answer to economic ills, France's Socialist government is planning for the highest deficit since World War II and proposing severe wealth taxes and special levies on high incomes.

In what President Francois Mitterrand regards as the most important accomplishment of his first 100 days in office, his government has announced nationalization and back-to-work programs. These measures, made public during the last 10 days, have made clear just how fast Paris and Washington are traveling in opposite directions in forming their economic policies.

It is startling to hear such strikingly opposite courses proclaimed by Mitterrand and Reagan as the best route to the same destination--a reinvigorated economy capable simultaneously of reducing unemployment and capping inflation.

Behind the contradictory approaches are philosophies and men no less contradictory: Reagan, the former actor with faith in the ability of American business to produce wealth for all the people and Mitterrand, the disabused intellectual who profoundly distrusts grand capital and believes government must control basic economic levers to ensure justice for the little people.

A prominent French economist, who clearly opposes Mitterrand's ideas, says, for example, that the president and his advisers in the Elysee Palace have set out to do more than raise money for the government with their wealth tax. They are, he explains, seeking to break up accumulated wealth--grand capital--in the belief that its redistribution will make the economy work better and more justly.

"They don't want to just resolve the unemployment and inflation problems," says the business-oriented economist, Jacques Plassard. "They want to change society."

Prime Minister Pierre Mauroy, describing his plans to fight unemployment to parliament this week, delivered a long, sober economics lesson followed by this peroration:

"The economic crisis will not solve itself. It will be overcome only with a formidable industrial adaptation and social mutation. It is up to the heirs of the first proletarians thrown at the foot of steam engines to achieve the emancipation permitted by new technology."

According to government estimates, the wealth tax will hit about 200,000 of France's richest families. Unless modified by parliament, the law will exact between 0.5 and 1.5 percent of fortunes above 5 million francs, or slightly more than $900,000.

Former finance minister Rene Monory, a bitter opponent of Mitterrand's policies, says the tax could force some businessmen to sell holdings to raise money to pay the government.

In an interview, he recalled what two provincial small businessmen, both in their fifties, recently told him: they are trying to sell out to larger corporations because of this fear.

"Just look around you," Plassard gestures at the restaurant where he is lunching with a correspondent. "The owner of this place probably has an apartment in Paris that is worth a million francs or so. A house on the Riviera, maybe, or in the country. Another million, maybe more. And the restaurant is worth several million. There you have the 5 million. He'll probably have to pay."

Executives who have little capital but high salaries also will be hit. To help pay a deficit in the government's unemployment insurance fund, Mauroy plans a one-time income surtax tentatively scheduled to strike couples whose combined salaries reach around $20,000.

The wealth taken out of these hands, in Mauroy's portrayal, will flow into the hands of those who need it more. Within a month of coming to power May 10, the socialist government raised France's minimum wage by 10 percent to the $3-an-hour mark. At the same time, it raised welfare payments to the elderly and handicapped by 20 percent, to just over $300 a month.

A month later, family payments went up by 25 percent and rent subsidies for poor families by the same amount, with another one-fourth increase pledged for Dec. 1.

The infusion of money--estimated at nearly $3 billion , which is likely to produce a $17 billion budget deficit--was followed this month by relaxation of credit rules.

Mauroy says he is counting on his stimulation of the economy to help reduce unemployment, which has reached 1.8 million, or 7.5 percent of the work force, as well as to help poor families keep up with 13 percent inflation.

In addition, he has pledged to hire 61,000 more government employes. For the longer term, Mauroy is urging part-time work in government ministries, widespread sabbatical leaves, a reduction in the work week to 39 hours, early retirement and job-training programs including scout-like work camps for youths just out of the Army.

In all, government officials predict that the steps will create more than 120,000 new jobs by next year.

For the more distant future, the government is putting much of its faith in the nationalization of a dozen key companies and 36 major private banks, bringing 95 percent of total deposits and about 17 percent of French industrial production under government ownership.

It is perhaps the nationalizations that best illustrate the clash in fundamental views between Reagan and Mitterrand on what makes an economy sound.

For Reagan, getting the government out of business is a major goal; for Mitterrand, getting the government into business is the major goal.

"America is returning, without embarrassment, to the original capitalism; it is taking a leap forward," wrote Louis Pauwels, a rightist author and journalist. "France is becoming socialist; it is making a backward fall whose seriousness we will soon be measuring."