Not a historian, but I've taken linguistics courses from a professor who worked in Africa. His observation was that language does not directly tie to ethnic/tribal identity in the same way that it does in the west. People are comfortable picking up new languages and incorporating them into their life without feeling like they are losing their cultural identity. Also, you'll find that most of the French speakers still also speak one or more indigenous languages as well.

The professor did field work in Sierra Leone, Liberia, Guinea, and South Africa that I know of.