Which language(s) is most useful for backpacking Africa?

I'm looking to backpack Africa in a few years, so I'd like to start learning a language that would the most useful/widely spoken. There are a few conditions I've conjured up though:

• the language should be spoken where English is not commonly spoken (else I would simply default to English, and rendering years of studying pointless)
• emphasis is given on languages in countries that can be visited. I realize that Africa is not as developed for tourism like SE Asia and South America are, so if certain countries are outright difficult to get to without paying a local guide, knowing locals, or paying exorbitant amounts of money, then it may not be so beneficial learning that language. Another reason why a country may not be able to be visited the presence of a major war -- though fully understand this is not at all predictable, I would be basing it just on current situations. I also state major because in lesser conflicts, certain parts of a country may still be accessible. This was the case when I visited Turkey.
• emphasis given on a language that is not broken up into many dialects. To clarify, initially I thought learning Arabic would be most useful, but upon further research, it doesn't seem like the best idea because of how many [incommunicable] dialects of Arabic there are (though reading and writing is still useful).

I realize the constraints may be a bit too strong, but any help is appreciated!