President Reagan has cited the effect of proposed federal of proposed federal bilingual education rules on Fairfax County as a prime example of federal regulation threatening to run amok.

Speaking here Monday to the National League of Cities, Reagan roundly denounced rules proposed last summer by the Carter administration that would have required schools to teach non-English-speaking students in their native language until they have mastered English. The proposal was rescinded by Reagan's education secretary, Terrel Bell, less than two weeks after the new administration took office.

If the proposal had been adopted, Reagan charged, its mandates would have been "impossible to fulfill."

"In Fairfax County, Virginia, for example," the president said, "students come from 50 different language backgrounds, 15 of which are spoken by more than 20 students. Were it able to follow the . . . guidelines, the county would incur the expenses of sponsoring bilingual programs in 15 different languages, including Urdu, Hindi and Laotian."

By citing Fairfax County the president followed a long line of critics of bilingual education who have used data from Fairfax to buttress their case.

For five years the county strenuously fought an effort by the federal government to get it to add native language instruction to its program of special all-English classes to help children who don't speak English at home.

In late December, less than a month before the Carter administration left office, it approved Fairfax's program as an acceptable alternative to bilingual classes under an old set of guidelines that Bell later said the Reagan administration would continue temporarily to enforce.

But the Fairfax county school system remained a vociferous and widely quoted foe of the proposed new bilingual rules.

"We're encouraged by the president's position," assistant county school superintendent Joseph L. King said yesterday. "That's been our position in Fairfax County for a long time."

Advocates of bilingual education seemed much less encouraged. But a spokesman for one bilingual group, Liz Benedict of the Mexican-American Legal Defense and Educational Fund, pointed approvingly to a part of Reagan's statement in which the president said he thought it was "proper that we have teachers . . . who can get at [non-English-speaking children] in their own language."

"We agree that children who don't speak English desperately need teachers who can speak their language," Benedict said, "and that's part of a bilingual program."

Reagan ended his remarks, however, with a declaration that "it is absolutely wrong and against [the] American concept to have a bilingual education program that is now openly, admittedly dedicated to preserving their native language. . . ."

Benedict said that wasn't the goal of the type of bilingual classes that her group supports.

But Fairfax's King said, "We don't think it's the public school's responsibility to [preserve] other languages. Our responsibility is to get kids ready to handle the mainstream of the curriculum in English, and we do that by teaching them in English. . . ."