There's actually no such language as "Celtic". Celtic refers to a group of dozens of languages, six of which are spoken today:
Subho ratri............But that is beyond formal.....We just say GoodNight in daily conversation.
Aloha po
You can say "tenha uma boa noite".
In Thai, you can say "ราตรีสวัสดิ์ที่รัก" (raatree sawatdii thii rak) to mean "goodnight my love." The phrase combines "ราตรีสวัสดิ์" (raatree sawatdii), which means "goodnight," with "ที่รัก" (thii rak), meaning "my love." It's a sweet and affectionate way to bid someone goodnight.
You would say, "Good night,I love you".
You can say "lafi" to say goodnight in Mandinka.
In Berber, you can say "tanemirt" to say goodnight.
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.
In British English, you can say "goodnight" or "night" as a shorter version.
In Sondhi, you can say "ດີ່ນກັນ" (dien kan) to say goodnight.
goodnight is "bonne nuit" in France.