Well, I'm a girl, and I would say that some of the best ways to say goodnight to girl depends whether you're saying it to her or texting/messanging her. If messenging her, then don't say it too late, and if saying it to her, then a simple goodnight is all it takes.
In Berber, you can say "tanemirt" to say goodnight.
You can say "lafi" to say goodnight in Mandinka.
goodnight
goodnight!
"Goodnight".
In Fulani, you can say "Jaama wolof" to say goodnight.
In Pangasinan, you can say "napagtolak" to mean goodnight.
Well it depends. First say goodnight sweet dreams that kind of stuff and go on from there
"Shab bekheir" is how you say goodnight in Farsi.
In Polish, you say "dobranoc" to wish someone goodnight.
In British English, you can say "goodnight" or "night" as a shorter version.