NBC News is jumping in on Friday where CBS News plans to tread later this summer. According to the latest CBS News/New York Times poll, only 8 percent of the 1 ,300 Americans of voting age who were queried were clear about the Reagan administration policies in both El Salvador and Nicaragua. (We support the government in El Salvador and side with the rebels in Nicaragua). The confusion of the general public has influenced CBS News to prepare a documentary on the subject for airing soon.
Ironically, while CBS has been busy polling, NBC News has already almost completed its documentary, designed to help clear up the mysteries of US Central American policy: ''NBC Reports'' Summer of Decision . . . Central America (Friday, 10-11 p.m., check local listings). A preview was not possible because NBC plans to add up-to-the-minute late-breaking material until the hour of airing.
However, it was possible to see an early script which, although still being edited, purports to be basically the show that will air with any late news developments added.
Reported by Richard Valeriani and Robin Lloyd, with Robert Rogers as senior producer, ''Summer of Decision'' examines the historical legacy and the strategic and political options involved in the internal conflicts in Central America. The timely documentary focuses on three troubled countries - El Salvador, Nicaragua, and Honduras. Says Valeriani on the program: ''Central America is a mess . . . (and, it's less than) 1,000 miles from San Salvador to Brownsville, Texas.''
According to the script, there is a ''bloody, stalemated guerrilla war . . . in Nicaragua, an anti-America regime now under pressure by internal and external opposition; in Honduras, an American military build-up in the midst of some of the hemisphere's worst poverty.''
The documentary appears to be a straightforward attempt to get answers to sometimes disturbing questions of ambivalent, sometimes seemingly hypocritical official attitudes. Among those asked tough questions are Sen. Daniel P. Moynihan (D) of New York and Secretary of State George P. Shultz.
One of the most confrontational segments in the transcript reviewed comes when Valeriani during an interview asks Mr. Shultz if the US isn't ''doing the same thing in Nicaragua as the Nicaraguans allegedly are doing with the Salvadorean insurgents. That is, we're supplying material for an external force to attack a government.''
Responds Secretary Shultz: ''The US government is not doing anything illegal. . . . To abandon people who are fighting for freedom and economic development to communist forces . . . would be immoral. . . .''
In the summary prepared for broadcast, correspondent Valeriani concludes: ''Four Latin American countries - the so-called Contadora Group - are now trying to work out a regional settlement effect, trade peace in El Salvador for peace in Nicaragua. But they've made little progress so far. For the time being, however, the negotiating track is inconclusive. And the only real prospect is more fighting.''
Now if CBS News would only stop polling long enough to get its Central American show on the air, too, perhaps American TV audiences (who too often do not read the excellent newspaper dispatches from Central America) would learn enough to raise that 8 percent knowledgeable figure just a bit higher.