Not Translators.. There are three possible explanations which I would place above translators. The frames in question came from Operation New York ( frame 9:20) March of 1966.
One Answer is in Nov 1963 South Vietnamese experienced a coup. The American backed President of South Vietnam was President Diem was caught and murdered by his own military lead by General Dương Văn Minh. Subsequently loyalists to President Diem were hunted caught and either killed or detained as General Durona consolidated power. Units loyal to the new government wore red bandanas around their necks to symbolize which side they were on. So it's telling that this was a South Vietnamese soldier wearing the bandana, this is a possible reason.
Operation New York post dates the coup by about 14 months.
Another possible explanation was that the North Vietnamese used red scarf to symbolize their Young Pioneers. Perhaps the South Vietnamese soldier was wearing it as a trophy.
Finally. In 1968, which is a little later in the war from Operation New York soldiers wore a red scarf as a kind of peace symbol. Perhaps the frames were from a little later in the war and were put in with the film of Operation New York. This could explain it also.