Realignment of the major European currencies, which boosts the value of the mark and guilder inside Europe, should have the psychological effect of strengthening the mark against the dollar as well, according to American international money experts.

C. Fred Bergsten, former assistant Treasury secretary and now director of the new Institute for International Economics sponsored by the German Marshall Fund of the United States, said in an interview that the dollar is "substantially" overvalued and should be expected to decline further.

And economist E. M. Bernstein, president of EMB Ltd., a Washington research company, said that the realignment of the European Monetary System will work to the advantage of the United States by accelerating the adjustment of the dollar-mark relationship, which already had begun to take place.

Bernstein said that the dollar had soared more than 40 percent against the mark from August 1980 to August 1981. But since then, the dollar dropped about 9 percent against the mark, and another 3 percent in the wake of the EMS realignment, for a total of 12 percent off the peak.

Yesterday, however, in the erratic markets that followed the assassination of Egyptian President Anwar Sadat, the dollar shot up, as did gold. But market followers believe that this was a temporary reaction, and that the dollar is likely to continued to soften.

In the EMS realignment, the mark and the guilder were raised by 5 1/2 percent, while the French franc and lira were lowered 3 percent against the other European currencies, all of which are expected to float within a narrow band around their fixed relationships. Those currency prices have no formal relationships to the dollar.

But as Bernstein explained, the new and more realistic alignment of European currencies, recognizing the strong position of Germany, can be sustained with fewer sales of marks, and that factor alone reduces some of the market pressure against the German currency.

For some time, exchange market analysts have recognized that the U.S. dollar has been seriously over-valued, or, putting it the other way around, the mark has been too cheap, when it took just over 2 1/2 marks to buy one dollar. In 1978, the situation had been reversed, when a mere 1.7 marks could buy a dollar.

No one really knows what an "equilibrium" rate is, but as the dollar got overvalued and the mark undervalued, American goods began to cost more, and German exporters were able to make big inroads into foreign markets, especially into the wealthy Persian Gulf.

Thus, the U.S. trade surplus, which had been running at an annual rate of $18 billion in the first quarter of 1981, fell to about $4 billion in the second quarter. Bergsten predicts that there will be a U.S. trade deficit in the third quarter.

Looking at it in terms of the current account (which includes services as well as merchandise), Bergsten predicts that the U.S. surplus of about $5 billion in 1980 will be just on the edge of surplus or balance this year, and then swing into "the biggest deficit ever" in 1982, topping the $14 billion in red ink registered in 1977 and again in 1978.

Treasury Undersecretary Beryl Sprinkel also has forecast a swing into a current account deficit, without giving any numbers. But Sprinkel doesn't agree that a large U.S. current account deficit implies a weaker dollar internationally. Sprinkel says that the dollar rate depends on many factors, including the markets' assessment of the strength of the U.S. economy.