Swahili is the lingua franca across East Africa, understood by somewhere between roughly sixty and two hundred million people depending on how second language speakers are counted, stretching from the Indian Ocean coast to the Great Lakes.
English works fine in most tourist settings. But greeting a lodge server, thanking your guide, or attempting a market exchange in Swahili tends to shift the whole tone of an interaction, and mispronunciation is met with warmth far more often than judgment.