Chancellor Helmut Schmidt of West Germany, in oblique criticism of President Carter's leadership, predicted yesterday that the United States under the incoming Reagan administration will demonstrate a "new decisiveness" in world affairs.

Schmidt's comment, one of several by foreign leaders on the prospect of Ronald Reagan as president, reflected his widely reported assessment that Carter's performance in the last four years fell below requirements of the U.S. presidency and world leadership.

Although he has expressed these feelings several times in private, Schmidt's remarks yesterday represented one of his most explicit public statements on the matter.

"There is a new decisiveness in the American nation, and that certainly will make itself felt in the new Senate, the new House and obviously in the new White House and President Reagan," he said on ABC's Issues and Answers broadcast from Bonn. "There is coming up a new will for identifying oneself in America with the interests and purposes of the American nation. Some part of it certainly has to do with the hostage affair in Tehran and some other factors do play a role."

Schmidt, whose coalition was returned to power in elections last month with a comfortable majority, is due in Washington next week for an official visit to meet with Carter. At the same time, he also is scheduled to establish his first direct contacts with reagan.

Looking toward Reagan's assumption of power in January, Schmidt said he would welcome more assertive U.S. policies, despite West Germany's prominent role in the increasing power enjoyed and wielded by Western Europe and his close ties with the sovereignty-conscious French government of President Valery Giscard d'Estaing.

"Many in Europe want the United States to act as a leader," he said.

The need for U.S. leadership is particularly acute now, he added, because of a complicated international situation including Soviet intervention in Afghanistan, the Iranian-Iraqi war in the Persian Gulf, Iran's continued holding of American hostages, the Arab-Israeli conflict and "new worlwide economic stagnation."

In sharp contrast to Schmidt's attitude, the Socialist International ended a four-day conference in Madrid with an expression of fears that Reagan's accession to power will hinder efforts to limit nuclear weaponry and will offer encouragement to Latin American dictatorships.

The congress, attended by representatives from 50 countries, said the socialist movement will watch U.S. policy in Latin America under the Reagan administration as a litmus test for its attitude toward human rights and other key international issues, Reuter reported from the Spanish capital.

Another negative prediction about the effects of the incoming Reagan presidency came from Farouk Kadoumi, or Abu Lutf, head of the Political Department of the Palestine Liberation Organization. Kaddoumi, who functions as PLO foreign minister, told the Beirut magazine Monday morning that Reagan will increase the chances of war in the Middle East.

Expressing a different view, Israeli Prime Minister Menachem Begin said it is too early to tell how Reagan's election will affect prospects for peace in the region. Interviewed on NBC's Meet the Press, Begin also stressed that Israel plans to hold onto the entire city of Jerusalem, dismissing Reagan's campaign suggestions of possible "Vaticanization" of the Holy City.