Senegal has plenty of great "true" african culture, it has a good tourist base though as does Ghana, both are popular destinations because of culture, beautiful beaches, cheap prices etc. and great food ( senegal especially)
parts of senegal are modern , like Dakar, and over touristy like st. louis but you can find plenty of people who are tied to the land, and leading traditional way of life, farming, working in textile, playing the djembe ( african drum) Senegal is a muslim nation, although not the same as being in say Saudi Arabia or Egypt, some people are strict adherents, such as in Touba where the Mouride brotherhood is based, and some are more secular. yes French will be of great help there, so will Wolof, the language most people speak in everyday situations there.
There are plenty of oppurtunities also to help people there, but Senegal is still a poor country.
same can be said for Ghana, quite well developed in many areas. they do speak English but ull find its not your standard American English, also the Twi /Akan language will be very helpful. The fanti/ewe/ ga cultures are pretty interesting. I think you should do some reading up on both countries, try to follow some peace corps blogs for instance and you can glean so much information about which country may be for you. good luck