South African Foreign Minister Pik Botha confirmed tonight that a conference to discuss the United Nations peace plan for Namibia would take place in January; the Mozambican government disclosed that its capital, Maputo, had been chosen as the site for the talks.

Speaking on the evening television news, Botha also said that if the problem of distrust between the South African-backed internal political parties in Namibia and the guerrilla Southwest African People's Organization, could be resolved, then March "would be a feasible date for implementation of the settlement plan."

Mozambican Foreign Minister Joaquim Chissano said tonight that both South Africa and SWAPO had agreed on Maputo for the site. He said his government hoped the talks could begin before January.

Months of behind-the-scenes negotiations with the United Nations staff and five Western allies -- the United States, Canada, France, West Germany and Britain -- appear to have brought South Africa to the point of assenting to enforce the settlement plan, itself in the making 3 1/2 years.

The plan calls for a cease-fire in the 15-year-old bush war between SWAPO and South Africa in Namibia, a semi-desert country that Pretoria has administered virtually as a part of South Africa for more than 60 years under a League of Nations mandate that the United Nations revoked in 1966.

A conference that would lead to enforcement of the plan would be a major diplomatic success for the Carter administration, which initiated the efforts to draw up the plan.

However, three differences between South Africa and SWAPO are still publicly unresolved. Any one of these could be used by either side to delay the talks, which U.N. Secretary General Kurt Waldheim has proposed to run from Jan. 7 to 14.

The most crucial issue is whether the agenda should be confined to implementing the United Nations settlement plan or expanded to allow discussion of a constitution for an independent, black-ruled Namibia.

The U.N. plan calls for a cease-fire followed by a U.N.-supervised election. The winning party would form an assembly to write a constitution.

Drawing a leaf from the Zimbabwe experience, the South Africans deem it preferable to have a constitution agreed upon before elections. In this way, a moderate, multiparty system rather than the Marxist, one-party state South Africa fears SWAPO will set up, can be established.

"The conference is not confined to the discussion of implementation of Resolution 435 (the U.N. plan), but any other practical proposal can now also be discussed," Botha said tonight.

A second problem is South Africa's demand that the United Nations show impartiality between the internal parties and SWAPO, which the General Assembly has recognized as the "sole legitimate representative of the Namibian people."

Whether South Africa will insist on the United Nations taking some steps apart from the talks to prove its impartiality is not clear.

A third problem is who would participate in the talks. Botha said tonight that his government would "be present in the form of advisers, in the background."

"We are not going there to participate in any decision or talks affecting the constitutional future of the territory," he said. "That is a matter for the parties to decide."

Botha said that South Africa's highest official in Namibia would choose the political parties to go to the talks. The South Africans want these parties to attend in their own right and as equals of SWAPO while Pretoria takes up the role of "front-line state" for itself.

SWAPO has insisted that it will not speak to the internal parties, which it regards as "puppets" of Pretoria, but only to South Africa itself. Chissano said Mozambique "considers this to be a conference between SWAPO and South Africa."

There are reports that SWAPO and South Africa have worked out a compromise in secret on this problem.