SOUTH Africans have seen much of their old system of government demolished. On Dec. 20 they may see the beginnings of a new system. That's when most of the country's political organizations will gather to begin negotiations on post-apartheid constitutional structure.The political viewpoints represented will run from those of the long-ruling National Party to those of the South African Communist Party. But preliminary meetings have indicated that the varied parties have enough common ground to proceed. At the least, they agree that a new polity must rest on democratic participation by all South Africans. Radical black- and white-power groups have effectively isolated themselves from the process. They can still make trouble, but they can't turn back what is clearly a majority quest for peaceful change. A central point of debate during the Dec. 20-21 Convention for a Democratic South Africa could be the African National Congress's demands for an interim administration and an elected constituent assembly. President Frederik de Klerk has dug in against the interim government proposal, seeing it as a capitulation. But there may be ways of involving a broader range of South Africans in national decisionmaking while maintaining the present administration of government services. The idea of electing the individuals who will write a new constitution has appeal. Such an election would begin the enfranchisement of blacks, and it would give the assembly's work added legitimacy. It could, of course, also raise hard questions about how to run an election before a new electoral law is worked out. These and other hurdles can be cleared, however, given the momentum behind the negotiating process. That momentum could ebb and flow. The government's negotiating energy may flag a bit whenever it loses a parliamentary by-election to the pro-apartheid Conservative Party, as happened recently in a small rural district. And the threat of violence in the black townships remains. But the country's course is set. And Dec. 20, even if it proves a largely ceremonial occasion leading to more substantive talks later, could be the date of embarkation.