The official language of Kenya is English, which means the government and the education is conducted in English. The students I met who wanted to further their education were spending long hours studying nonfiction books in English, as well as reading stories, in order to prepare for their school exams. (You can see a video taken while I was there which shows children sitting in a “classroom” under an acacia learning English rhymes. I also interviewed a number of the younger people in English.) Thus, a good grasp of English is absolutely necessary for furthering your education in Kenya.
The national language is Swahili, which means that is the common language of the marketplace. It can also be heard informally in the school setting.
Then there are a number of tribal languages, including Kikuyu, Luhia, Luo and Kikamba, as well as others spoken by smaller groups. In the region where the camel-borne library operates, many though not all of the library patrons speak Somali.
Donors have the opportunity to contribute books in English (including some on an Amazon wish list written by Kenyan or Somali authors), as well as books in Swahili here or here and in Somali here or here. When I visited last year, the library carried books in English or Swahili. It did not carry any books in Somali, but I have been told that some of our donors sent books in that language.