Formal:
* Penblwydd hapus i ti, fy annwyl. (Pen-blooth hap-us ee tee, my ann-dwill) - This is a very formal and polite way to say "Happy birthday to you, my dear."
Informal:
* Penblwydd hapus, cariad! (Pen-blooth hap-us, car-ee-ad) - This is a more informal and affectionate way to say "Happy birthday, love!"
* Penblwydd hapus i'r cariad gorau yn y byd! (Pen-blooth hap-us ee'r car-ee-ad gor-ow in ee bid) - This translates to "Happy birthday to the best love in the world!"
You could also consider:
* Dymuniadau gorau ar eich pen-blwydd! (De-moon-ee-ad-oo gor-ow ar eich pen-blooth) - This means "Best wishes on your birthday!"
The best way to say it depends on your relationship with the person and how you want to express your affection.