"Oes rhaid i ti ddweud?"
Translation:Do you have to tell?
Perhaps a more common way of phrasing it in English would be "Do you have to tell?".
A: Dw i'n mynd i ddweud wrth yr heddlu. "I'm going to tell the police."
B: Na! Oes rhaid i ti ddweud? Plis paid! "No! Do you have to tell (them)? Please don't!"
My scriptwriting skills aren't the best, but you get the idea!