This will be an English question.
In singular, you can either say "You are Tanzanian" or "You are a Tanzanian", as the first "Tanzanian" is an adjective and the second is a noun.
Well, the pluralized corresponding sentences would be "You (all) are Tanzanian" and "You (all) are Tanzanians". Am I right?
I ask this because "You are Tanzanian" was detected as with a typo, and the correct version was "Tanzanians". And I am not an English native speaker, also.