WHEN the developing-country debt crisis exploded in 1982, many importers of United States goods in Latin America had to cut back sharply. They had no money. Now the buyers are coming back."While developments in Russia are important from the geopolitical standpoint, they are dwarfed in the economic sphere by what's happening in Mexico and Latin America generally," notes Sam Nakagama, a Wall Street consulting economist. US exports to the Soviet Union in the first half of this year comprise only 0.8 percent of total US exports. That compares with 7.4 percent for Mexico, America's third-largest export market after Canada (No. 1) and Japan (No. 2). Richard Feinberg, an economist with the Overseas Development Council in Washington, D.C., sees a potential for more exports to the Soviet Union (which appears to be breaking up) when it gets its economic house in better order. This year, though, sales to the Soviet Union are down 18.9 percent, while exports to Mexico, at $15.6 billion in the first half of 1991, are up 14.9 percent above the year-earlier pace. Sales to Brazil, at $2.6 billion, were also up 14.9 percent; sales to oil-rich Venezuela, at $2.2 billion, rose 60 percent; and exports to Argentina, at $785 million, increased 69.6 percent. "Under new, more vigorous leadership, markets in Latin America are opening up and US exports are beginning to show sizable gains," writes Mr. Nakagama. Mexico, Argentina, and Brazil have programs, of varying success, for liberalizing their economies. Further, US exporters are enjoying increasing success in East Asia. Exports to Singapore are up 25.1 percent to $4.8 billion in the first six months of this year, sales to Thailand up 25.2 percent to $1.9 billion, to Malaysia up 19.6 percent to $2 billion, to Hong Kong up 9.4 percent to $3.9 billion, to China up 18.5 percent to $2.9 billion, to South Korea up 8.7 percent to $7.8 billion, and to Taiwan up 6.9 percent to $6.3 billion. US exports to South Korea are already as large as those to France. "A diversified group of developing countries are very important trading partners to the United States today," says Mr. Feinberg. "The US has important stakes in an open global trading system including worldwide trade talks now under way. Though the latest statistics indicate the 1990-91 recession in the US lasted through the second quarter, vigorous growth in exports helped keep the slump relatively mild. Earlier this week Commerce Department statisticians also noted that the nation's merchandise trade deficit shrank to $15.6 billion in the April-June quarter. That's the smallest gap in eight years. US exports climbed 3.2 percent to an all-time high of $104.1 billion. Imports grew a tiny 0.4 percent to $119.7 billion. For the first half of 1991, the trade deficit ran at a seasonally adjusted annual rate of $68 billion, 37.1 percent lower than the $108.1 billion deficit for all of 1990. Some analysts believe the trade deficit will worsen again as the US economy picks up, sucking in more imports, and weakness abroad in some export markets trims demand for US exports. Contrariwise, Nakagama sees grounds for optimism for US exports. Exports to Canada, down 2.5 percent this year, will surge once the Canadian economy picks up steam, he figures. The same is true of Britain, Australia, and Spain, he says. And further liberalization in Latin America should help. Feinberg hopes the external debt problems of Brazil and Argentina can be resolved within a year or two by a "Brady plan" deal involving debt reduction, debt rescheduling, and domestic economic reforms. Such a deal, named after US Treasury Secretary Nicholas Brady, was earlier worked out with Mexico. And as for the US economy, opinion sways between optimism and pessimism. "Compelling reasons suggest that a more accommodative Federal Reserve policy is needed," states Lacy Hunt, chief economist, HongkongBank group. He sees monetary and credit restraints worsening, inflationary forces subdued, dismal corporate earnings, shaky durable goods orders, and high initial unemployment gains. By contrast, Nakagama anticipates "more signs that the economy is doing better than many had expected."