What are facts about lingua francas in Tanzania?

Swahili has been a lingua franca in East and Central Africa for a thousand years and more. A lthough it's a second language to most Tanzanians and Kenyans, it has the status of national language, so it's not really a lingua franca anymore. It's also an official language of the Demcratic Republic of Congo, where it is the first language of almost everyone in the east, from Lubumbashi in the south to Lake Kivu in the north. It was brought to the Congo by Arab and Swahili slave-traders in the 19th Century. In the enormous disruption of peoples and cultures that was caused by the slave trade, Swahili replaced local tribal languages as a first language.