PRESIDENT REAGAN'S trip to Europe next May was beginning to confront him with diplomatic questions of greater complexity than he cared to cope with. He was to be in Bonn from May 2 through 4 for the seven big industrial democracies' annual summit conference on economic policy. That much was settled. But what about May 8, the 40th anniversary of the end of World War II in Europe? That's where the planning got stickier. Last June the Western allies of that war had conspicuously omitted the Germans from their commemorations of the Normandy invasion, and the German government was not anxious to have the snub repeated. But Mr. Reagan did not care to risk any gesture that might have unintended implications in Europe's complex and unfamiliar internal politics. Precisely where was he to go, and what message was his presence there to convey?

The White House has now arrived at a deft solution. On May 8 Mr. Reagan will address the European Parliament in Strasbourg -- in France, but on the German border. Strasbourg and the parliament symbolize a great many things that an American president can enthusiastically support without fear of getting entangled inadvertently in the domestic divisions of the countries that he is visiting. Strasbourg, which has been under both flags, now stands for reconciliation between France and Germany -- the end of a longstanding and immensely destructive quarrel. Created by the postwar movement to unify Western Europe, the parliament there has become the Common Market's legislature. It hardly has the importance of the national parliaments, but, particularly since the recent introduction of direct elections, it is by no means insignificant. As a point on Mr. Reagan's itinerary, the parliament at Strasbourg will stand neither for the war nor for the victory of 1945, but for the slow and patient work in the postwar years to tie the former enemies tightly together.

There were other visits that Mr. Reagan might usefully have made. He could have gone, for example, to the concentration camp at Dachau as a gesture of support to all those Europeans -- many Germans prominent among them -- who are determined not to let time diminish the terrible lessons of the Nazi period. That would not have been a day misspent.

But Mr. Reagan has chosen to follow the practice of other American presidents and avoid the sites that evoke the most painful recollections. Presidents are usually optimists who prefer not to dwell, at least in public, on the dark side of politics and human nature. Mr. Reagan is going to Europe to celebrate the industrial democracies' present unity and the world's hope of prosperity. Strasbourg is the place for that.