The Reagan administration yesterday named Otto Juan Reich, a foreign-aid specialist, to be an ambassador in charge of promoting administration Central America policy with the U.S. public.

Reich, 38, an assistant administrator for Latin America at the Agency for International Development, takes over the roving ambassador's job left vacant by former Florida senator Richard B. Stone when he became President Reagan's special envoy to Central America.

Reich's new post involves making speeches, writing speeches and articles and "generally stroking opinion makers" in an effort to build support for U.S. efforts in the region, according to a State Department official.

In an interview, Reich said he expects to be "a behind-the-scenes information gatherer and disseminator" for the media, civic organizations and foreign governments. "We find a great lack of information generally," he said.

A former Washington director for the Council of the Americas, an association of businesses with interests in Latin America, Reich is an old friend of Stone, who reportedly recommended Reich for the political post.

Reich will report directly to Secretary of State George P. Shultz and have the "personal rank" of ambassador, the department said. The job is not subject to Senate confirmation.

State Department spokesman Alan Romberg, announcing the appointment, said Reich's role "will be in public-policy efforts rather than in formulation" of either public or private diplomatic plans.

Other officials said that means Reich's task is to deliver bluntly to the public the administration's warning that further leftist advances in Central America, especially in El Salvador, pose unacceptable dangers to the United States.

The move comes as polls show that most U.S. voters are misinformed about U.S. policies in Central America or deeply suspicious of them.

It also presages what is expected to be a major administration effort to win additional funding for its military and economic aid programs in El Salvador and to forestall cutoff of its covert aid to rebels opposing the leftist government of Nicaragua. Important congressional votes on both issues are expected later this month.

The administration also announced that Stone is to leave Thursday on another trip to Central America, stopping first in El Salvador. White House and State Department spokesmen declined to say whether meetings are scheduled with members of the Salvadoran left, who have asked repeatedly to talk with him.

The administration continues to describe Stone's role as that of facilitating contacts between the left and the Salvadoran government to bring the left into the political process there.

Reich's conservative views are expected to mesh easily with those of the rest of the State Department's new Central America team, which includes Stone, Langhorne A. Motley, new assistant secretary of state for inter-American affairs, and Thomas Pickering, a career diplomat nominated as ambassador to El Salvador.

Reich, born in Cuba and fluent in Spanish, joined AID in 1981. Educated at the University of North Carolina and Georgetown University, he was Army liaison officer with the government of Panama between 1966 and 1969, a period of widespread unrest there, and has held several other jobs concerned with Latin America and the Caribbean.