Schmidt explains Reagan to Brezhnev

''Interpreter'' is the new buzzword in West German-superpower relations. This is the phrase West German officials have finally settled on to describe Chancellor Helmut Schmidt's role in explaining the Reagan administration to his Soviet guest, President Leonid Brezhnev - and in explaining the Kremlin to Washington.

Thus, Schmidt is said by his spokesman to be convinced that both Washington and Moscow want serious negotiations about nuclear arms control - and that both are ready to move toward each other in negotiations. He is said to be telling Brezhnev this in their conversations Nov. 23 and 24. His envoy will presumably carry the same message to Washington as soon as the Soviet-West German talks are completed.

The conservative opposition here sees any role as superpower interpreter as a presumptuous one for a West German chancellor. The Americans and Russians can talk to each other any time they want to, opposition politicians argue, and certainly do not need the Germans to explain the two to each other.

Nonetheless, the fact remains that for months the Americans and Russians did not talk to each other at policymaking level. The Reagan administration took a very long time even to appoint and send an ambassador to Moscow. And it was only at the prodding of the West Germans that the two countries' foreign ministers finally got together a few months ago - and that superpower negotiations will begin on European nuclear arms control Nov. 30.

Moreover, a West German chancellor is in a position to exchange views with a Soviet leader in a vigorous give-and-take of a sort that is not possible for an American President. United States-Soviet summits have always been reserved for celebrating agreements that have already been reached (or for bringing the pressure to bear that will force the last minute concessions needed to wield final agreements.)

Meetings of the Soviet and West German leaders, by contrast, can be exploratory in nature. And they might even have some impact on Soviet thinking, given the Soviet respect for German organization, stability, and know-how, and for Schmidt's own stature within Europe.

Certainly the Schmidt-Brezhnev dailogue is nothing but exploratory. No agreements are being signed by the two leaders. Brezhnev is not here on a state visit, but on a working visit. And what repeatedly filters through the veiled briefings about the official dialogue is Soviet questioning about what really makes the Reagan administration tick.

The Russians have always had difficulty in making any sense of the chaos they see in Washington; to this day the whole Watergate affair remains a mystery to them. And Reagan in some ways has seemed the biggest puzzle of all.

Past experience (especially with former President Richard Nixon and former Secretary of State Henry Kissinger) had led the Kremlin to believe that it was easier to do business with a Republican than with a Democratic administration.

To be sure, superpower antagonisms would be clearer and polemics sharper, but Republican administrations would be more ''realistic'' and ''business-like'', than Democratic ones. And once the two sides did strike a deal on the basis of this realism, the Republicans would be able to carry the deal politically with the American right wing (as the contrasting examples of SALT I under Nixon and SALT II under former President Carter showed.)

The new Reagan administration, however, did not play according to the old rules. The polemics continued, but there were no businesslike negotiations - and not even any high-level conversations - in the background.

The Kremlin has utilized this hiatus brilliantly to appeal to antinuclear movements - and encourage anti-Americanism - in Western Europe. But it has never figured out what to do with the central superpower relationship.

Pending the resumption of direct US-Soviet negotations Nov. 30, then, the Russians have been acutely interested in hearing West German thinking on this issue.

One of the minor footnotes of the Schmidt-Brezhnev summit is that no Schmidt-like role as interpreter is enjoyed by the West German and Soviet press spokesmen. At every opportunity the Soviet information and propaganda chief Leonid Zamiatin has engaged in polemics both against the US and against his West German counterpart, Kurt Becker - to the surprise of Becker and the irritation of the journalists.