THE FRENCH local elections turned into a triumph for just about everybody who had managed, one way or another, to run against President Valery Giscard d'Estaing. In the first round of voting last Sunday the Gaullist right under Jacques Chirac conflicted a humiliating defeat on the President's chosen candidate for mayor of Paris. Elsewhere the left cleaned up. The Socialist-Communist alliance won slightly over half the votes cast nationwide. Even the environmental movement won some seats. The green candidates, as they were called, did well enough to justify apprehension within a government that has committed itself to build the Superphenix, a gigantic breeder reactor on a fully commercial scale.

True, there's a tendency among French voters to enleash the radical impulse a bit more freely in this kind of politics than in national elections. But the bend is unmistakable. As usual, there's more than one explanation. For one thing, Mr. Giscard e'Estaing's government is paying for various examines to inept administration - not least the case of Abu Daoud, the terrorist who was picked up by police and then, on orders from above, hastily released. For another, an air of now-or-never urgency has united the right wing. But underlying all the rest is the state of the French economy.

France's inflation has dropped sharply since Premier Raymond Barre imposed his plan last September. But unemployment is still very close to its highest point of the recession, and in fact it has been there for a full year. The forecasts of French business expansion this year are not very promising.

It raises an unhappy possibility that France's tribulations will now become circular. The more powerful the left becomes, the more frightened the businessmen get and the more reluctant to invest. A low level of investment only perpetuates the high unemployment which, inevitably, further strengthens the left.

The next great opportunity for the left will be the national parliamentary election in the spring of 1978. It seems certain to be an event of immense importance not only for France but for Europe as well. The Italian Communists, who could defeat the fragile Andreotti government whenever they choose, have made it clear that they do not intend to move until they see how next year's French election comes out. Spain's Communists are now operating openly as the country works its way carefully toward a more democratic rule. The West Germans, sticking firmly to the middle of the road, look with rising dismay on the advances of the radical left in Latin Europe - and they worry, more intensely than ever, about the possibility of isolation between the Russians on one side and the Eurocommunists on the other.

How significant is a local election? Sunday's ballots were, after all, only to choose town councilors - 450,000 of them, from Paris to the most remote rural hamlet. The smallest towns, those of 100 inhabitants or less, elect nine councilors; the French believe in a generous measure of full representation, and you do not have to be a close student of French life to suspect that ideology may not be the only influence on village voting. But, after all the qualifications have been stated, it's quite clear that the French voters have delivered a sharply adverse judgment on their present leadership. Power is now balanced sufficiently precariously in western Europe that every tremor has to be read as a hint of the great decisions to come next year.