Hendrik might understood Dutch because his parents were Dutch and Protestant missionaries. But also the Afrikaners used the Dutch bible version till 1933. After 1933 the Dutch version translated into Afrikaans.
Dutch and Afrikaans are extremely similar languages, basically because Afrikaans is a derivative of Dutch.
Depending on how old he was when he left the Netherlands would indicate whether or not he had learned and could speak actual Dutch.
As he was only 3 months old when the family moved to SA, there is no way we can work out from his biography whether he did speak Dutch.
It is rather likely that the family spoke some Dutch in the house, most emigrant families seemed to do, but it no way certain.
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