I don't know about an official proverb, but the meaning would be, for example:

People are people.


I don't think 'the humans' nature' would work here (and if you want to use it, it's '_ human_ nature'.)

If I understand the German correctly, the corresponding phrase in English is:
"You can't change human nature."

Oh, OK, Gibson. "People are people" means about the same thing as far as I'm concerned. My suggestion is the more proverbial sounding thing to say. And you're right, of course. No apostrophes.
Typically used as a kind of resignation about the lesser qualities of human nature.

No, I like yours. It does sound proverbish, doesn't it. So it's probably better for Mrs Beels requirements.

Your both comments are of great help! I think I go for "you can't change humans' nature".
Many thanks!

No, not humans'. Just 'human'. See #2 & #3.

got it. :)