Good night

And when people get back home and leave the office, they say "Good Night", which for me means "Bonne Nuit". Sounds strange.
Is this more appropriate than "Good evening"?

I noticed this in English but not in other languages. But "Good night" is only said when saying goodbye to someone, either because they're going back home, or going to bed.
It is incorrect to greet someone "Good night", even if it is the middle of the night.

"Good evening" is the opposite. You cannot say "Good evening" to someone when you are going away. It is exclusively a greeting. It is incorrect to say "good evening" to say goodbye to someone, even if it is in the evening.

Also,

bonjour = "Good morning" if it is in the morning and you are greeting them for the first time that day.
= "Hello" otherwise, or if you don't know the person yet.

You can say "hello" in the first case, but "Good morning" would be the most appropriate. "Good day" (= "Bon Jour") is only used in Australia.

vince said:

You cannot say "Good evening" to someone when you are going away. It is exclusively a greeting. It is incorrect to say "good evening" to say goodbye to someone, even if it is in the evening.

However, you can say " good evening" when saying goodbye.

And, I believe at some time in history, 'good evening' was used to say goodbye. I think it still is used now by certain people, or in certain regions. But it is very rare in my part of the United Sates.