Definitions

Official language: a language which by virtue of its legal status is used for official nation-wide communication in administrative, political and legal matters.

National language: a "dominant language in a multilingual environment used for regional or even nationwide communication", potentially sharing some of the functions of the official language (H.E. Wolff in Heine/Nurse 2000, African Languages. An Introduction. p. 320)

Countries where Swahili is spoken

Tanzania Swahili is an official language, alongside with English
Kenya Swahili is a national language, alongside with other major African languages
Uganda Swahili is widely used as a language of interethnic communication
Democratic Republic of Congo Swahili is one of four national languages

Other countries: Swahili is also used in Rwanda, in Burundi (for commercial purposes), and by a significant number of people in the southern part of Somalia and in northern Moçambique. It is also encountered in the larger cities of Southern Ethiopia

Dialects

There are many regional varieties of the Swahili language. Standard varieties used for teaching and in the media in Tanzania and in Kenya differ only slightly. The difference between these varieties and the standard used in Eastern Congo is, however, much greater.

Use of Swahili in public life

Swahili is used in all spheres of everyday life, including television, radio and newspapers. In education, it serves as a medium of instruction up to secondary school level. Its rich literary and artistic tradition, which goes back to the 18th century, has kept up with modern trends in oral art and in writing