The Fula language is spoken over a large area of West Africa. The major dialects known as Pular, Pulaar and Fulfulde are considered by Ethnologue to be separate languages, but most linguists consider Fula to be a single language. Wilson (1989) is quoted on Wikipedia stating that "travellers over wide distances never find communication impossible". There are roughly 14 million speakers in 17 African territories. Over a million speakers live in each of Guinea, Senegal, Nigeria, Niger and Mali.
Reference: Wilson, W. A. A. (1989). Atlantic. In John Bendor-Samuel (Ed.), The Niger-Congo Languages, pp.81-104.