These are all the African countries Mary and Greg might ride through. Communication should prove to be interesting with such a vast diversity of languages. Where there are percentages behind a language, it denotes the percentage of the population which uses it for their primary language. You'll notice that while a lot of countries have English as an official language, it isn't heavily used in some of those countries.

Country Flag Language(s)
South Africa Zulu (official) 23.8%, Xhosa (official) 17.6%, Afrikaans (official) 13.3%, Northern Sotho (official) 9.4%, Tswana (official) 8.2%, Tsonga (official) 4.4%, English (official) 8.2%, Sotho (official) 7.9%, Tsonga (official) 4.4%, Swati (official) 2.7%, Venda (official) 2.3%, Southern Ndebele (official) 1.6%, other 0.5%
Lesotho Southern Sotho (official), English (official), Zulu, Xhosa
Botswana Setswana (national) 78.2%, Kalanga 7.9%, Sekgalagadi 2.8%, English (official) 2.1%
Zimbabawe Shona 76%, Sindebele/Ndebele 18%, English (official) 2.5%
Zambia English (official), Bemba/Chibemba, Lunda, Lozi/Silozi, Chinyanja, Tonga, Luvale, Tumbuka, Koanda, and about 70 other indigenous languages
Malawi Chichewa/Chinyanja (official), Chiyao, Chitumbuka, English (official), and others.
Tanzania Kiswahili/Swahili (official), English (official), Arabic, and many local languages
Rwanda Kinyarwanda (official), French (official), English (official), Kiswahili/Swahili
Uganda English (official), Ganda/Luganda, other Niger-Congo languages, Kiswahili/Swahili, Arabic
Kenya Kiswahili/Swahili (official), English (official), numerous indigenous languages
Ethiopia Amharic (official) 32.7%, Oromo 31.6%, Tigrigna 6.1%, Somali 6.0%, Guragigna 3.5%, Sidamo 3.5%, and other local languages.
Sudan Arabic (official), Nubian, Ta Bedawie, diverse Nilotic languages, English (official, but rarely used)
Egypt Egyptian Arabic (official)