This is a group name, not the name of one language. Nobody speaks a language called Bantu; people speak Swahili, Zulu, Shona, and hundreds of others that scholars put in the same family because they build words in similar ways. The word is uncountable in this use, so you do not say a Bantu or two Bantus about languages. In practice you will see it most often in front of another noun: Bantu languages, a Bantu language, Bantu speakers.

In this use the word names people, and it is countable: a Bantu, two Bantus. That is where the trouble sits. A name built for a language family gets put on human beings, and it covers hundreds of separate groups with one word, as if they were one. Some people find that offensive. So the safe move when you are writing is to name the actual group, or to say Bantu speakers or a speaker of a Bantu language, which describes what someone does rather than labelling who they are. You will still meet this use in older books and old official papers.