PARIS, May 11

Linguistically speaking, it was a stab in the back. As President François Mitterrand was mobilizing diplomatic and political energies to prevent French from losing its privileged position in the new Europe, one of the nation's most prestigious organizations, the Pasteur Institute, decided to publish its research bulletins under English titles. An obscure administrative decision by the institute's director, Maxime Schwartz, was abruptly transformed by outraged French intellectuals and the press into another setback for an uphill cause: securing the position of the stately language of Corneille and Racine in the fast-moving, frontierless European Community that should come into being at the end of 1992. Perhaps only in France would the head of state take a personal interest in the decision to change the title of Les Annales de l'Institut Pasteur to Research in . . . - followed by the disciplines of immunology, virology and microbiology - for three separate publications that together have fewer than 5,000 subscribers around the world.

Truth in Packaging

A dastardly plot against the French language and against Mr. Mitterrand's vision of making France a high-tech mecca? Not as Mr. Schwartz tells it. ''In 1988, we received 249 manuscripts, of which about half came from French-speaking countries,'' he said. ''Nonetheless, only 6 percent of them were written in French.'' The newspaper headlines suggested that the bulletin was being turned overnight into a Trojan horse of the perfidious Anglo-Saxons. Yet the title change was basically a truth-in-packaging measure: the vast majority of the articles inside have for years been in English.

Though beleaguered, the French language still holds some important ramparts in Europe. It remains the lingua franca of the European Community headquarters in Brussels, itself a French- and Flemish-speaking city; it is the magistrates' language at the organization's Court of Justice in Luxembourg, though plaintiffs may bring cases in nine languages. The community's daily news briefings in Brussels are conducted in French, and there are belligerent shouts of ''Français! Français!'' if a journalist poses a question in English to a native English-speaker who happens to be briefing.

French, Proper and Otherwise

After the late-comers Britain and Ireland joined the community in 1973, their envoys to Brussels found themselves in a club where the rules, documents and even the jargon had for two decades been shaped in French. The English-speakers have for the most part adapted. ''One of the main differences between French and English is that French has more rules, whereas English operates by nuance,'' observed Adrian Fortisque, a senior community official, who is British but speaks fluent French. ''It is very easy to take a string of well-honed French formulae, link them together with proper grammar, and you have a perfect community document.'' Some French expressions are less proper, as Prime Minister Margaret Thatcher and her aides discovered during a contentious European summit gathering in February 1988. The British Prime Minister's unbudging determination to cut agricultural subsidies prompted an obscene outburst from Jacques Chirac, who was then France's Prime Minister.

Even French-speaking Britons in the room had to consult their dictionaries to determine just how gravely Mr. Chirac had insulted Mrs. Thatcher. The next day The Sun, a flamboyant British tabloid, demanded in a banner headline, ''Say Sorry Rude Frog.''

Concessions Regularly Made

Although French is entrenched as the tongue of the Brussels mandarinate, concessions are regularly made to English. Jacques Delors of France, the president of the community's executive branch, had to work hard to upgrade his school English to understand the monolingual Lord Cockfield, the British architect of the 1992 single-market blueprint.

De Gaulle spoke decent German but poor English and occasionally conducted his legendary feuds with Churchill in French; one confrontation became so heated that the two leaders asked their interpreters to leave the room. Shortly after being elected President in 1974, the patrician Valery Giscard d'Estaing raised titters in Paris when word leaked that he was taking English lessons; his deep friendship and alliance with Helmut Schmidt, the West German Chancellor, had been consolidated not in German or French, but in English, while both were Finance Ministers.

The French political establishment is increasingly knowledgeable in English. Mr. Chirac speaks competent English, which he solidified while working as a dishwasher at a Howard Johnson's in Cambridge, Mass., in 1953. President Mitterrand wields the French language with a cadence and power matched by no other politician, but he speaks no other language.

Pen Is Mighty Defensive Sword

In 1984, Mr. Mitterrand created the grand-sounding Commissariat General de la Langue Française, which was assigned to coordinate official and private efforts to ''defend'' the French language. Metaphors of siege and battle recur almost obsessively when the French talk about French. ''We have not declared war on any foreign language,'' maintained Bernard Billaud, the entity's commissar general, while complaining of ''Franglais,'' as English inroads into French are called. Five months ago, he issued the fifth edition of the Dictionary of Official Neologisms, which instructs bureaucrats on which French expressions to use in the place of English intruders.

The dictionary contains 2,400 words that are supposed to be avoided or adopted. ''Jumbo jet'' is ''gros porteur,'' ''spot'' is ''message publicitaire,'' ''scoop'' is ''une exclusivité,'' a ''one-man show'' is ''un spectacle solo,'' ''boom'' is ''boum,'' and ''fast food'' becomes ''pret-a-manger.'' Mr. Billaud has even arranged for defenders of the language to tap into France's omnipresent telephone-linked computer network to zap the pervasive Anglicisms that have crept into French. Yet the commissar general sounded a bit weary of the battle.

''The other day I asked Prof. Luc Montagnier of the Pasteur Institute if he would inform the world in English if he discovered a cure for AIDS,'' Mr. Billaud said. ''His answer was, 'Of course.' ''