I'll be darn, I didn't know that.
"Black South Africans were at times officially called "Bantus" by the apartheid regime. Nowadays in Southern Africa, the term "Bantu" is no longer used to refer to a people. The more common and polite term is "Black" and in fact legislation and documents from the South African government replaced "Bantu" with "Black". "
Is it okay to say they speak a Bantu Language?
I suppose so if you wince properly when you say it.