Bruno Druchen, the national director of the Deaf Federation of South Africa, said the unnamed man seen on television next to leaders including US President Barack Obama "was moving his hands around but there was no meaning in what he used his hands for".
Four sign language experts said the man was not signing in South African or American sign languages and could not have been signing in any other known sign language because there was no structure to his arm and hand movements. South African sign language covers all of the country's 11 official languages.
The man also did sign interpretation at an event last year that was attended by South African president Jacob Zuma. At that appearance, a deaf person in the audience videotaped the event and gave it to the federation for the deaf, which analysed the video and submitted a formal complaint to the ANC.
In their complaint, the federation suggested that the man should take the five years of training needed to become a qualified sign language interpreter in South Africa. But the ANC never responded.