Do people need nitrogen from air for health?
Do people need nitrogen from air for health?

No. This is well established!

Can people breath totally nitrogen-free atmosphere for a long time?

Yes. Also well established!

I know, nitrogen is essential for life, and in big quantities, but
maybe people can take it entirely from food, from proteins, etc?

As expounded on in the comments by AliceD, nitrogen gas (N2) is entirely taken in by humans from their diet, since atmospheric N2 is pretty inert. It is also not necessary for gas exchange to take place in the lungs.
Some extra questions as an addendum:

Can one substitute N2 in atmospheric air with another gas, and sustain human life?

Yes. All side effects would be attributable to the effects of the substituting gas (e.g. it may be heavier or lighter or equally or more reactive than N2).

Can 100% O2 sustain human life?

No, and such a case has never been recorded. Oxygen toxicity following hyperoxia is very dangerous. Ironically, too much oxygen damages lung tissue and epithelial membranes via oxidation and render the lungs unable to exchange gases correctly; and so death by suffocation follows in such extreme cases such as with 100% O2.