France is likely to follow a radically different policy in Africa but would continue traditional policies in other areas of foreign relations if the left wins this month's parliamentary elections.

Talks with Socialists involved in foreign affairs and French and other Western diplomats reveals a consesus - that a Socialist-led government would switch alliances in Africa from conservative to "progressive" governments but could be expected to continue the current Gaullist policies regarding Europe, nuclear strategy and East-West relations.

The thinking that is going on about the foreign policy of a leftist government is based on the increasingly likely looking presumption that the Communist cannot afford, politically to prevent a leftist victory and that Communist good behavior at the polls is all that it will take to allow leftist victory.

Should the left win a majority of parliamentary seats, the question of relations between President Valery Giscard d'Estaing and a leftist government in Paris is still something of an abstraction. It also seems that the practical working of the government that would emerge in such a situation would involve compromises and adjustments.

The constitution gives the prime minister and his cabinet control of policy but makes the president of guarantor of the country's institutions. The ambiguity has been used by Gen. Charles de Gaulle and his successors to make foreign and defense policy the "reserved domain" of the president.

The type of foreign policy departures contemplated by the Socialists will undoubteldly prompt Giscard's resistance. But it is too early to predict how the two sides would work out their differences.

The most spectacular immediate change sought by the Socialists would be, according to insiders, a raprochement with revolutionary Algeria, including an offer of French arms and an attempt to make a new agreement to purchase more oil for energy-short France.

This would involve abandoning Giscard's backing for Morocco's King Hassan and his underpopulated ally Mauritania in their war with Algerian-organized guerrillas over the Spanish Sahara.

The Socialists say privately that they would fully expect the Gaullists and Giscardists to attack such a new African policy as an abandonment of the network of interests in French-speaking Africa that President de Gaulle so carefully created to maintain France's influence in its former empire.

Giscard's staff is already saying that the Soviet Union is positioning itself in Africa so that it could fill the void if the French were to abandon their traditional allies.

While the Soviets are described as exercising extreme caution in former French Africa, there is general agreement among Western diplomats that the Cubans are already involved in the Spanish Sahara conflict with a training mission at Tindouf, the staging area in Algeria for the Polisario guerrillas who have been infiltrating Morocco, Mauritania and the disputed Western Shhara territory.

Elsewhere in Africa, the Socialists say privately that they would:

Withdraw as fast as possible the 4,000 French troops from newly independent Djibouti in the Horn of Africa. The French force is the only meaningful Western presence left in an area where the Soviets and Cubans are implanting themselves with large numbers of troops.

Drop the present government in Chad. The French, bound by a military cooperation agreement, have 300 men in Chad training and aiding the government forces against Libyan-backed northern rebels supplied with Soviet arms. Giscard sent one of his closest aides recently to try to get the Chad government to accept the loss of the largely desert northern half of the country in exchange for peace in the arable southern half, but Chad reportedly resisted the idea.

Establish close ties with President Sekou Toure of Guinea, the only leader of a former French colony in black Africa who rejected de Gaulle's friendship. The Socialists say that they are even willing to risk cooler relations with Western-oriented Senegal and the Ivory COast, which are on either side of Guinea and are the crown jewels of France's former west African empire, to get closer to Sekov Toure.

Propose sanctions against South Africa and pursue a militantly anti-apartheid policy going beyond Washington's.

The Socialists are already on record as wanting to renegotiate all of France' existing military agreements in Africa. Privately, they say they want ot drop commitments to give military help to governments threatened internally, a common practice from De Gaulle to Giscard.

The Socialists are also publicly committed to such gestures as breaking off diplomatic relations with the military government of Chile.

But on matter closest to home, French foreign affairs observers say they often have a hard time distinguishing between the present government's policies and Mitterrands'. If anything, Mitterrand seems to be even more Gaullist in some of his thinking and terminology than Giscard.

The Socialist leader has spolenm out against the current system of freely floating currency exchange rates as a gimmick to reinforce the U.S. econimic hold on the Western world.

Mitterrand has also said he would not hesiate to use whatever escape clauses there are in the European common market ot protect the French economy if it runs into any difficulties over the left's costly social and economic commitments such as substantially raising the minimum wage and family and old-age allowance.

Both Western and French diplomats say they fear that a leftist government might be forced to become extremely protectionist because of the economic effects of its woen domestic social programs.

One of the few areas where Mitterrand has made an absolute break with Gaullism is to pledge French partcipation in all the international isarmament talks that de Gaulle boycotted. The Socialist leader is also reliably said to be determined to block the current French nuclear reactor deal with Pakistan, Iraq and South Africa.

Ironically Mitterrand is far less enthusiastic about France's independent nuclear strike force than the Communists and the left wing of the Socialist party. They see the force as a guarantee of French independence from Washington.

Communists leader George Marchais yesterday reproched Mitterrand for pleding that there would be national referendum on continuation of the strike force.

Giscard challenged Mitterrand from the opposite flank this week, saying that France's unclear firepower will be tripled by 1985.

It was one of several recent statements by Giscard laying the groundwork for a claim to maintain the direction of foreign policy even under a leftist government.