Overall, using "du" is acceptable in 90% of social interactions. It's only when talking to people in higher positions or as a courtesy to elderly / unfamiliar people.
This holds for private life. In professional life (except for IT industry and design industry; both are much more casual in the interaction style), it is rather "Sie" is acceptable in 90% of social interactions.
That is pretty much exactly how nigga is used.
In English, there is no spec for different levels of formality. There is no universally documented way to being less courteous to bosses and policemen. Saying "sup dude" or "how's it going" is part of the American standard library, not the language.
And to your point directly, "nigga" is incredibly informal and casual, and would never be something anyone (much less a non-native speaker) should ever use unless they know what they are doing. And usually, they should be black, too.
Nigga is just "bro" or "dude" but exclusively used among black/minority communities. I realize the word nigga is A Big Deal for white people but really in our community it is used as casually as the word "like" in any given sentence. Interestingly I have even used it with black bosses before in a joking casual way.
The same is not even remotely true of bro or dude
It's so common people joke about it "don't call me dude, bro | don't call me bro, pal" etc etc