"We are Tanzanian."
Translation:Sisi ni Watanzania.
Ninyi is the second-person plural; when you refer to multiple people directly (like the "you" in that sentence!)
Sisi is the first-person plural; in regards to multiple people, including yourself, being in a group and being affected in the sentence (this sentence above would say that both you and the group around you, usually defined before the sentence, originate from Tanzania).
If this sentence was "Ninyi ni Watanzania," it would be stating the nationality of those around you while leaving your own nationality ambiguous; you could be Tanzanian without saying it, or something else i.e. British, American, Tunis, etc.