MORE THAN 400 international observers, including former President Jimmy Carter, have agreed to monitor the balloting on Dec. 16. SUSPECTED IRISH Republican Army guerrillas threw a bomb studded with nails at a Belfast police station early Saturday, injuring two policemen. . . . PEACE TALKS BETWEEN the Somali government and five rebel groups scheduled to start in Cairo Tuesday have been postponed indefinitely, Egypt said Saturday. . . . FORMER PRESIDENT Ronald Reagan and his wife Nancy left Britain Saturday aboard a private jet for California after a five-day visit.