BOUTROS BOUTROS GHALI, chosen last week by the Security Council to become secretary-general of the United Nations, personifies his new post.Mr. Ghali, an Egyptian, is a Coptic Christian, fluent in Arabic, English, and French. His wife is Jewish. He was instrumental in negotiating the 1978 Camp David accords and has managed to remain a moderate and a peacemaker - not easy in the Middle East. The UN General Assembly is expected to confirm Ghali, and should. He will take charge of an organization whose influence has steadily increased in the past decade under the leadership of Javier Perez de Cuellar. Mr. de Cuellar deserves praise for his tireless diplomacy in El Salvador, Cambodia, the Middle East, and many other places. If world events continue on their present course, the UN will become even more important. The bipolar world of superpower confrontation has ended. More than ever, the UN will have to step in to mediate regional conflicts. Never have there been more blue-helmeted UN peacekeeping troops stationed abroad than at present - 10,000. That figure will increase with the UN's unprecedented peacekeeping effort in Cambodia. Yugoslavia will be an immediate concern for Ghali. The Security Council is working on an oil embargo there. Ghali is also expected to seek ways of easing the debt burden of developing nations. Other tasks for the new secretary-general: * Reform of the Security Council - there's talk of broadening it to include Japan and Germany. And what about the Soviets' permanent seat now that the USSR is fragmenting? * The UN's role as a safety valve for crises in Central and East Europe. * Striking a balance between sovereignty and human rights - an issue raised by the plight of Kurds in Iraq. When should the borders of a sovereign state be crossed in order to stop genocidal actions or other affronts to international law?