"Fraglais" has returned to White House menus, along with hard liquor. Until the Carters arrived, French words intermingled with English and cocktails were served before dinner. They changed all that.

If the Carters had been entertaining British Prime Minister Margaret Thatcher, last night's roast rack of lamb persilees would have been roast rack of lamb with parsley. Vegetables printiere would have been springtime vegetables. But now that the Reagans are in the White House it's back to the kind of menu that promed one French journalist to write: "J'attends d'etre invited at the White House. J'y speakerai English with M. Nixon without difficulties."

Guests at the Reagans' first official dinner for a visiting head of government were offered mixed drinks, mineral water and wine before they were served the most classical of menus. Nancy Reagan made all the decisions about the dinner. "It's the first time," said longtime White House executive chef Henry Haller, "a first lady is so very much involved. We had to cook a complete tasting lunch for a few people. Mrs. Reagan made a few changes. I was very pleased the first lady had so much interest."

So much interest meant adding a fresh mint sauce to the lamb, changing the potato croquettes to potato fleurons (they are shaped like chocolate kisses) and removing the sauce that is usually served with a hot Grand Marnier souffle. Mrs. Reagan, as she explained earlier this week, wanted a "well-balanced menu so that you don't have everything creamy. I just thought it was too much with the first course," pompano in champagne, which has a sauce. Mrs. Reagan also asked for more vegetables in the vegetables printaniere.

The wines: Beaulieu Vineyard Pinot Chardonnay, Inglenook Cabernet Sauvignon 1974 and, for toasting and the 30 after-dinner guests, Shcramsberg Blanc de Noirs. The Inglenook alone costs about $15 a bottle. Mrs. Reagan said the president does not involve himself in dinner preparations. But he likes good wines, and only American wines will be served during his administration, a custom followed by the Carters.

For last night's dinner, Mrs. Reagan worked with her social secretary, Muffie Brandon, choosing the tablecloths, china and silver. "Once you decide on the tablecloth," Mrs. Reagan explained, "the rest just sort of evolves." She selected green moire cloths, a mixture of china, gold vermeil flatware and, for centerpieces, gold baskets filled with red, pink, blue and white anemones. Gone are the porcelain birds, fruit, vegetable and flower arrangements and the pieces of driftwood.

"I love to do tables," Mrs. Reagan said.

"But," she added, as she tucked her legs under her on the couch in the Map Room and launched into her campaign, "the White House needs some more china. There hasn't been any china bought for the White House since the Johnsons and that's the one with the state flowers on it. It's very pretty, but. . ."

But it isn't quite formal enough.

For last night's dinner, the Theodore Roosevelt, Franklin Roosevelt, Wilson and a little Truman china were "carefully mixed, 36 of this, 60 of that," Brandon said.

Dinner, at 12 round tables, was for 96 people. Mrs. Reagan doesn't want any more than that number, though official dinners in the past have often been for 120. "If you go to the trouble of making a pretty table . . . and people can't see them because it's so crowded. . ." And, she added, service is better now because the butlers can get around the tables.

Mrs. Reagan, noted for her attention to detail, has instituted a new custom so that the official visiting party, which passes through the receiving line first, is not left along to stare around the empty dining room. Last night the British delegation went into the Blue Room for three or four minutes while some of the American guests made their way through the receiving line to the dining room and were there to greet them.

It's what Muffie Brandon calls "a grace note. Grace notes are what make or break the hospitality or kindness," she said.

That kind of hospitality, minus the setting and the flowers, would cost $55 a person if a caterer provides the food, wine, liquor and the butlers to serve the meal.