In Tagalog and Thai, Vietnamese, Cantonese and 28 other languages, the nation's radio waves are singing the message: "Answer the Census. We're counting on you."

Rosalyn Carter, Cesar Chavez, Kirk Douglas, Elvin Hayes, Mickey Mouse and a score of other stars are telling prime-time television audiences: "You can count on us!"

It's all part of a promotional extravaganza designed to convince 222 million Americans, many of whom are distrustful of the government, to fill out a form full of nosy questions from the U.S. Census Bureau.

The billion-dollar headcount, scheduled for April 1, is the most controversial, complex and politically charged census in history. Last time, in 1970, some 5 million Americans -- 2.5 percent of the population -- weren't counted at all.

This time the government is trying hard to do better. But not hard enough to suit big-city mayors, whose federal handouts depend on census numbers, or politicians whose districts are apportioned with census data.

The 1980 census is "a bureaucratic nightmare," Atlanta Mayor Maynard Jackson charged in a congressional hearing yesterday. The undercount will be no better than in 1970, he predicted, "cheating us out of millions and millions and millions of dollars."

Rep. Benjamin Rosenthal (D-N.Y.) criticized Census Bureau director Vincent Barraba for relying on a voluntary rather than a paid media campaign, and for promising and then not providing local officials enough time to review the count.

Barraba, however, said the census will get $40 million worth of free advertising -- more than it would have received with a paid program. He said local officials will have two weeks to review the counts, and census officials another six weeks to ferret out anyone they might have missed.

The 1970 undercount disproportionately affected large cities, especially in minority neighborhoods. Census takers were reluctant to venture into high-crime areas. Apartment complexes are difficult to enter and many people -- for example welfare cheaters, illegal aliens or missing fathers -- simply prefer not to be counted.

The bureau estimates it missed 1.5 percent of the white population and 7.7 percent of blacks. In the case of black men between 25 and 34 years old, one out of five was not counted.

A similar margin of error would be much more serious this time. Some $50 billion worth of federal programs, many enacted since the 1970 census, are keyed to the numbers. Jackson said Atlanta lost $11.7 million worth of employment funds and 6,000 jobs because of the 1970 undercount.

District of Columbia Mayor Marion Barry has said that the undercount cost the city $1.9 million in revenue-sharing funds in 1978 alone. Some estimates figure that for each person missed, a city or town gets $150 to $200 less a year in federal funds from school lunch money to highway aid.

Some towns are so desperately dependent on the census data that, according to one Census Bureau source, a local New Jersey official offered a bribe if the bureaucrats could come up with higher numbers.

While city-dwellers are worried about an undercount, Rep. Jim Leach (R-Iowa) is more concerned about an overcount. Census takers, appointed by Democrats, could pad the numbers, he fears, to increase urban representation in Congress at the expense of rural areas. Census officials say the computers would catch any fudging.

The National League of Cities and the U.S. Conference of Mayors have called on the Census Bureau to adjust the figures to compensate for the undercount -- a sort of statistical affirmative action. A National Academy of Sciences committee called for adjustment to apply to federal funds, but not to reapportionment. A Senate bill, sponsored by Daniel Moynihan (D-N.Y.), would require funds adjustments.

The bureau held a conference last month on the technical and legal aspects of adjusting for the undercount but most demographers are skeptical of the idea. "It would cause more problems than it would solve," a census official said. "Once you start playing with the numbers, you're still playing with the numbers."

Whatever the outcome, the undercount controversy is likely to spark a thicket of lawsuits. Mayor Jackson said yesterday that several groups are already thinking of challenging the 1980 census as unconsititutional, based on the "one man, one vote" principle. But if data are adjusted and districts reapportioned based on people who may or may not exist, and certainly can't be found, lawsuits would flourish, officials said.

The Census Bureau is also under attack for counting illegal aliens. A group called the Federation for Immigration Reform, joined by 26 members of Congress, was turned down by the Supreme Court this week in its effort to bar aliens from being considered in reapportionment.

The issue, however, is likely to come up in future censuses. Counting aliens, critics argue, gives disproportionate power to certain congressional districts, especially in the Southwest.

While illegal aliens and poor minorities may constitute much of the undercount, the Cenusus Bureau is worried about the post-Watergate, anti-government sentiment of the population at large. "There's growing distrust of government," says bureau official Mark Ferber. "It's not just the underclass, but the middle class too."

Presidential candidate Ronald Reagan has urged Americans not to answer most of the census questions. Surveys have found that 41 percent of Americans in 1971 and 48 percent in 1978 considered census-type questions to be an invasion of privacy.

Although the Census Bureau insists that all information will be held confidential for 72 years, as law provides, the American Civil Liberties Union wants to destroy all names after data are tabulated. Census officials argue that would betray history: Alex Haley used census data to research "Roots."

So on Census Sabbath and Census Sunday, if all goes according to plan, the priests and rabbis will preach from the pulpit, "Answer the Census." And in the next two weeks, restaurant placemats, lapel buttons, street posters, utility bill inserts, school lessons and 40 million Social Security checks will trumpet the same message.

In the barrios of Los Angeles and the tenements of Southeast Washington, more than 200 community workers will seek out people in pool halls and even on street corners. "It's like a Normandy landing," said a census official. "It's only going to happen once."