I was never taught the difference between mtanzania and watanzania. So lately it's just a 50-50 guess.
See my reply in this thread: https://forum.duolingo.com/comment/32432801
If in the mobile, hover over or tap the Kiswahili word for English word. Smae for website.
For anyone who is confused, Mtanzania is singular and Watanzania is plural.