Swahili is a fluid, evolving language with many dialects, not all of which are mutually intelligible.
When languages move across geographical spaces like that it’s not at all unusual to see a large number of dialects and versions spring up, and that’s exactly the sort of dynamic recent history Swahili has shown us.
Swahili has always been a flexible language, constantly evolving. Historically, there are three major divisions in Swahili which some even consider to be distinct languages, and which are, admittedly, not always mutually intelligible with Standard Swahili (which is based on the version spoken in “Stone Town”, or old town in Zanzibar).
The versions of Swahili that could be considered distinct languages are Kimwani, centred in the Kerimba Islands and the north coast of Mozambique; Chimwiini, spoken in small pockets on the south coast of Somalia; and Kibajuni, found on the Somali–Kenyan border and in the Bajuni Islands.
For the rest of the many dialects of Swahili, they all fall under three major umbrellas, all of which contain numerous sub-dialects with minor regional variations: Mombasa–Lamu Swahili (with dialects found in Lamu, on Pate Island, Somalia, Mombasa, Mafia Island, Zanzibar, Malindi, and Kenya); Pemba Swahili (with dialects spoken in Pemba Island, Zanzibar, and the Kilwa District; and Comorian (with dialects spoken on the Comoros Islands and Mayotte.
Interestingly, a sub-dialect in the Mombasa-Lamu family, Kingozi, is quite ancient and rarely used today, but is considered to be a possible parent language to Swahili overall.
Swahili is incredibly fluid, and always changing and borrowing, and new dialects have formed relatively recently as the language invades other areas. Some examples would be Kingwanai, which can be found in the south of the Republic of Congo, Shaba Swahili, sometimes referred to as Copperbelt Swahili, and Sheng. Sheng is fascinating because it’s actually a blend of Swahili, English, and a variety of ethnic languages spoken around Nairobi. It began life as a patois spoken in the slums but has become a fad among the fashionable in Nairobi, helping to spread it.
A final major dialect of Swahili has taken shape in Somalia as well. Called Chimwiini, it’s spoken primarily by the Bravanese people who live along the Benadir coast in that country.