"Bye"
* Hwyl fawr! (Formal, similar to "Goodbye!")
* Cwtsh! (Informal, meaning "Hug!")
* Hwyl! (Informal, literally means "Enjoyment!")
* Ffarwel! (Formal, literally means "Farewell!")
"Good"
* Da (General, meaning "good")
* Dda (Formal, meaning "good")
* Hwyl fawr! (Can also mean "Goodbye, have a good time!")
So, depending on the context, you could say:
* Hwyl fawr! (for both "bye" and "good")
* Cwtsh! (for "bye")
* Da (for "good")
It's important to remember that Welsh is a living language, and there are various ways to say "bye" and "good" depending on the formality and region.