Kirundi or Rundi is a Bantu language with about 4.5 million speakers, most of whom live in Burundi. There are also some speakers of Kirundi in Uganda, Tanzania, Rwanda and Congo-Kinshasa. Kirundi is closely related to more or less mutually intelligible with Kinyarwanda.
There has been a standardised spelling system for Kirundi since the 1940s, though Roman Catholic and Protestant missions use slightly different systems.
Kirundi is one of the official languages of Burundi, along with French, and is the spoken mainly by Hutus, but also by Tutsis and Twa people, in most parts of the country.
source
wikipedia.org,
omniglot.com &
ethnologue.com