Couldn't it be that there is not as yet a rule as to whether to capitalize the nationalities in Tanzanian, but that the fact that in English they MUST be is making the translator feel unsure as to whether he should or shouldn't capitalize them in Swahili?
I don't know how to explain it, but the following explains it well. It's a noun class. "M" basically means singular, "wa" means plural. See if this helps: https://en.wiktionary.org/wiki/Appendix:Swahili_noun_classes
It's like in Asian languages. They have particles meaning "person", and particles meaning "plural/several of this"
So person + country = nationality.
Ex: person + England = English (person)
And plural + country = several persons of the nationality
Plural + England = (some) English (people)
Assuming you're new here, it depends. Duolingo gives vocabulary and practices so you won't forget. You cannot always depend on it though. It would be better if you use Duo along with some other apps or websites. 'Memrise' is one. Other than that, a visit to Wikipedia now and then(regarding the grammar and structure) would certainly help.
My advise is: persevere. Pay attention to the little changes in the forms provided in the language you are studying and try to relate them to the changes in meaning. i.e. mmarkani= american, wamarekani = americans. There you are, m singular, wa plural, probably. And so on. Good luck. Also Posting your doubts, comments and questions here we all get to communicate with each other and share our progress.