Fairfax County Board of Supervisors voted yesterday to mount a legal challenge to proposed federal regulations that would require public schools to teach basic subjects in the native language of foreign-born students.

Opposing what one board member criticized as one of the "most mindless" federal government programs yet developed, the supervisors authorized the county attorney to work with the school board to prepare a legal challenge to the regulations if they are adopted by the U.S Department of Education.

Virginia Attorney General J. Marshall Coleman already has threatened to file suit to block imposition of the regulations, calling them an "unwarranted intrusion" on states' rights. Mandated bilingual education could have a major impact on Fairfax and Arlington counties, where a growing number of students speak Vietnamese, Spanish and Korean.

Fairfax County school officials said there are 2,700 students in the school system who use English as a second language. They complain that the federal plan, suggested last July by Education Secretary Shirley M. Hufstedler, would require the county to hire instructors to teach history and math in 50 different languages.

Board members argued yesterday that the bilingual plan would delay assimilation of foreign students into an English-speaking environment. And they defended the county's own "English as a Second Language" program, which costs $1 million a year and teaches the students English first before moving them into regular classes.

The federal education office is still soliciting public comment on its proposal prior to drawing up a final plan.