Translation of "May peace prevail on earth"
Tangnefedd ar y ddaear
"Tangnefedd" means not only the absence of war, but also social harmony, inner tranquillity and spiritual peace. For the Welsh it has a meaning deeper than merely the absence of armed conflict.
In speech "Tangnefedd" possibly could be used as a greeting, but most definitely can be used as a farewell.
I update translations as times change and the way people express things change.