Perhaps the proficiency of Swahili has been overestimated. Recently, our church here in Kondoa visited Choka, a Rangi village about 12 miles north. We attended a service of the handful of local Christians, and one of us was asked to lead the joint prayer, so he started by saying in Swahili ‘Fumbeni macho’, (“Let’s close our eyes”). The villagers seemed confused. Only when someone translated into Rangi ‘Chuumbi miiso’, did they understand. Consequently, the preacher interspersed his Swahili sermon with a lot of Rangi explanations.
This experience was a real eye-opener, especially for the urban Tanzanian Christians who had expected Swahili to be much more widespread in the countryside. And there are 150 Rangi villages around Kondoa Town.