A DELEGATION from the Japanese Diet will be here in Washington this week, engaging in the usual careful and skillful political reconnaissance. They will be particularly interested in American intentions regarding trade and Japanese imports. These conversations follow a familiar pattern. The Americans dwell on the tremendous disparity between eastbound and westbound trade between the two countries, and warn that truly terrible things will happen -- an uncontrollable political reaction -- if it continues. The Japanese, who have been hearing these same warnings continuously for 15 years, nod solemnly and go home reassured that nothing much has changed.

Perhaps both Japanese and Americans can do a little better this time. The real concern here is Japan's failure to take its full share of responsibility for the stability of the international trading system -- a trading system that is absolutely vital to Japan's own prosperity. To be specific, three points:

First, economic growth rates. The American economy has slid into a period of low growth. That's the time when the other major economies -- Japan and West Germany -- ought to speed up. Both are anxious about their internal deficits and are running in second gear. Both can afford to shift into high. It would serve their national interests, and the world's.

Next, money. Thrift is a virtue, up to a point. But Japan, a nation of assiduous savers, is now sending its savings abroad -- mainly to the United States -- in amounts that have wildly skewed the exchange rates. It's time for the Japanese to begin investing more in their own country -- a rich country that fails to provide a service as basic as sewerage to most of its people, and leaves cherished amenities such as park land in extraordinarily short supply.

Third, the Latin American debts. As a country with enormous surpluses of capital to export, Japan could make a historic contribution to Latin development. Instead of buying dollars and supporting American overconsumption, the Japanese might more usefully buy pesos, cruzeiros and australs. The risks would be greater, but the returns to Japan and the world would be greater still -- and not in the narrow financial sense alone.

The present imbalance of world trade, with the gigantic Japanese surpluses and even more gigantic American deficits, is totally instable. Those surpluses and deficits can't be sustained indefinitely. Everybody knows that. The strongest industrial countries have the largest obligations to show courage and initiative in working toward a better balance. That means the United States. It also means Japan.