Does anything about the sentence indicate that the definite article 'the' should be used in English? Because I tried it without 'the" and it wasn't accepted.
There is good reason not to accept "People of Tanzania are nice". When the sentence stands alone as it does in this exercise, one should opt for the most general solution, namely "The people of Tanzania are nice." You need a context for dropping the article. For example, if you are in a conversation about people from various countries, your friend might observe that people from Finland are reserved, that people from Germany are dogmatic, that American people are aggressive, that Tanzanian people are nice, implying that some are, referring to observable trends. Then the context would be sufficient. In English, one could then say "The Tanzanians are nice" or "Tanzanians are nice" and the meaning would be the same, the context dictating that you mean some, many or most, but not all. Very many languages don't have articles. If you study a few of them, you quickly begin to have a sense for the basic redundancy of articles in languages. We shouldn't really have any need for them.
When do you have to translate with "of" or a genitive? or is there no difference?
Because its in beta, many of the phrases are transliterated. People of tanzania vs Tanzanian people for example