As Africa is a very diverse continent, there is no one answer. Since you already know English, I would suggest that learning French would give you the best return on investment of your time, because it's an official language in many African countries and it's relatively easy for an English speaker to pick up. Also note that Portuguese is quite useful in the southern part of the African continent.

from my experience, in term of official languages French is the most spoken official language in Africa(Spoken in 26countries), 2sd : English(20countries), etc. About african dialects, "Swahili" is the most spoken, from Uganda, Kenya, Tanzania, Zanzibar island, Comoros, Rwanda, Burundi, la Somalie, la Zambie, DR. Congo, Congo even in south Africa, you'll find swahili speakers. Hope my contribution will help you, because I travelled in 11 different African countries, so...good luck in your searches.

Arabic is used at north of Africa.