Goodbye! I will miss you!" in Welsh is "Hwyl fawr! Byddaf yn dy golli di!"
Goodbye: there are several ways of saying this. "Da bo ti" (familiar) and "Da bo chi" (formal) are the usual ones. There is also "ffarwel", which to be honest I have only ever seen in print. If you...
You may get away with: Cymro ac yn falch (Welshman and proud) or Cymraes ac yn falch (Welshwoman and proud)
Draig Cymraeg
Hwyl FawrYn iach! Ffarwel. Hwyl fawr