THE political tremors shaking Africa out of its long tradition of one-man rule centered this week in Zaire. President Mobutu Sese Seko has been in office for 26 years and for much of that time has been viewed as an ally of the West in its battle against communist expansion.Also during that time, Mr. Mobutu amassed a fantastic fortune, estimated at $3 billion, much of which is stashed in bank accounts abroad. He is thought to have regularly skimmed as much as 50 percent of the country's income for personal and patronage uses. Those days are gone. Back in August, the president sensed the political tide turning and said he would introduce multiparty democracy. He legalized a few opposition parties and agreed to a national conference to debate the country's future. His critics assert, however, that Mobutu has thwarted the conference at every turn. Meanwhile, Zaire's economic nose-dive has been hastened by a decline in the price of copper, its prime export. Public payrolls went unmet, which led to the current rioting by soldiers. Mobutu may have hoped that Western forces would arrive to bolster him again, as in the past. The French and Belgians have sent in small contingents, but only to evacuate their nationals. These old allies of Mobutu's now join in calls for democratic reform. As in the Soviet Union, such reform has sparse foundations in Zaire. Constitutions have come and gone during Mobutu's reign, but the rule of law and respect for human rights had no chance to take root. The seeds of democracy now have to be nurtured, and in this huge African land of 38 million ethnically diverse people - as in the former communist East - international aid and encouragement is needed. Zaire's private sector and institutions of government could use an infusion of outside expertise - given n ot from the old colonial motivations, but in the spirit of democratic brotherhood. Ultimately, Zaire's people - like those in other African countries experiencing the pull of democracy - will have to assert their sovereignty and choice of government. They'll also have to avoid the kind of ethnic and tribal regressions that have beset others trying to throw off tyranny.