Nichelle Nichols frequently tells a story of getting into a dispute with director Marc Daniels over the filming of this episode. As it had already been established that Uhura’s first language was Swahili, Nichols believed that, after her mind was erased, Uhura would revert to her first language. However, as Nichols herself did not speak Swahili, Daniels wanted Uhura to just speak English. Nichols refused to, telling Daniels, “Nichelle Nichols doesn’t speak Swahili, but Uhura does!” Gene Roddenberry was eventually brought in to settle the dispute, and he sided with Nichols. A linguist specializing in Swahili was then brought in to write the few lines of Swahili that are spoken in the episode. (Memory Alpha)
there’s a special place in my heart that occasionally just swells with love for TOS.