The borders you see today are not necessarily drawn based off which group of people live where. But more remnants of colonial administrative districts. Two different peoples speaking two different languages will fall back on the common one, regardless of its origins. Therefore the use of ex-colonial languages.

http://www.metmuseum.org/toah/ht/11/sfc/africa_1914.jpg Notice the similarities? Ethiopia, Somalia, Cameroon, Angola, The Congo, Sudan and a number of coastal west african nations are basically the same as they were drawn by the colonists. Here specifically the 1914 partitions.

Here is a very detailed ethno-linguistic map with which you can zoom in on, compare it with the borders drawn in 1914, and those present on the map, or a modern one. http://www.chaz.org/Arch/Turkana/Namoratunga/Geography/Linguistics_large.jpg Even the "uniform" Green section denoting Guinean languages is fractured as each colour represents a language group, rather than a language. There is no guarantee of decent intelligibility between two languages. People speak english, French, etc because it the administrative language before independence and it was the only one with which they could seriously communicate between ethnic groups that spoke different languages.

This is somewhat of a generalization however because recent changes in borders are changing the trend. For example, in this old map, the Sudan is a single country. The country split into two in 2011. As you can see, the southern part of Sudan which split was dominated by a different ethno-linguistic group than the North.

EDIT: Added a map, elaborated a little.