For a Japanese prime minister to urge his country publicly, on television, to buy foreign goods is an extraordinary departure from tradition. Most Japanese have always felt that it's not only safer to buy Japanese but, generally speaking, a kind of patriotic obligation as well. That's the presumption that Prime Minister Yasuhiro Nakasone is now trying to change. He is quite right to warn his people that their engrained habit of spending their money at home has become dangerous to them.

Mr. Nakasone's courage is beyond doubt. The question is whether he and his government can move fast enough to deflect the sense of grievance and the impulse to retaliate that has seized Congress.

American and Japanese negotiators have worked through the latest list of disputes, and in response Mr. Nakasone announced a series of measures to open Japan's markets a little wider. These measures will surely be helpful, but they seem unlikely to make any dramatic difference. The Japanese government will, for example, increase its financial aid to its wood products industry with the thought of lowering the tariff on foreign competition in two or three years. Why not sooner? Well, the Japanese say, you have to understand that the wood products industry is in bad shape in Japan and politically the subject is very sensitive. That's the kind of answer that enrages American senators, who take incessant pounding from American industries under pressure from Japanese imports. Why should Japan's inefficient plywood manufacturers be sacrosanct, when American automobile producers face rising imports? The Japanese government has been very slow to acknowledge that these cases of outright protectionism -- plywood and baseball bats, oranges and beef, so forth and so on -- have an inflammatory effect all out of proportion to their economic importance.

But the reality is that total American sales to Japan will depend mainly on the dollar-yen exchange rate. The Reagan administration's mismanagement of the American economy and the resulting American interest rates have lifted the dollar so high that no Japanese concessions can make much difference until it comes down.

A division of responsibility suggests itself here. The United States bears the main responsibility for the exchange rate and for the scale of the American trade deficit. The Japanese government bears the responsibility for resolving the protectionist irritations -- plywood and all the rest.

On the Japanese side, Mr. Nakasone has made a brave beginning. In the interest of both countries, let's hope that he has the stamina to keep going. It's doubly important because, unfortunately, on the American side there's not much prospect for action on economic policy soon.