Nos da, William.
BTW the Welsh version of William is Gwilym.
nos da
Nos da i chi. Good night to you.
You can say "lafi" to say goodnight in Mandinka.
In Berber, you can say "tanemirt" to say goodnight.
Because at the end Willie goes to live with Mr Tom Oakley forever and he can say, every night, Goodnight Mr Tom.The name of the novel comes from something William said early on in the story. Up to this point William had been addressing Tom as "Mister". Mr Oakley tells William you can call me Tom. William immediately replies "Goodnight Mister Tom"
goodnight
"Goodnight".
goodnight!
In Fulani, you can say "Jaama wolof" to say goodnight.
In Pangasinan, you can say "napagtolak" to mean goodnight.
shab bekheyr
In Polish, you say "dobranoc" to wish someone goodnight.