It is certainly not essential to be able to converse in both languages but it does help. More important, is the fact that it is always helpful to try and spea and understand the main languages of the area in which you are working, as there are still Afrikaaners and Black African people, who struggle with English.
I seem to think that Afrikaans tops the most used language. I remember reading that a vast amount of the Coloured community use Afrikaans as their main language and, since the modern Afrikaans language evolved from the time that Dutch was banned by the British, from being spoken, the Coloured people, defying the ban, carried on their form of Dutch in their homes and in the back streets and alleys. This new form of Dutch was adopted by most of the younger people, when the ban was lifted. I may be wrong.