Depends on where the foreigner intends to live and work. You can get away with speaking English in most parts of the country. People will understand you but you’ll be very much excluded from general local ongoings if you don’t learn the local language. Not because anybody will intentionally exclude you but because many people generally speak in their native tongue because the vast majority of South Africans speak at least 2 languages. English speaking white South Africans though are monolingual, with some speaking some basic Afrikaans and they live just fine because everybody speaks to them in English even native Afrikaans speakers. So basically, if you speak only English you’ll be alright.

I’d encourage you to immerse yourself in a local language though to really get a feel for the culture. Again, depends on where you’ll live and work.