"Who is the youngest?"
Translation:Pwy yw'r ifanca?
What is the difference between: "Pwy yw'r ifanca?" and "Pwy ydy'r ifanca?"?
yw and ydy are equivalent. Which one you use is entirely optional, although in different areas of Wales you will tend to find one used more often than the other. In the media you will hear and read both. Just pick one and be consistent in using it.
(They are both colloquial, shortened forms of a more formal form ydyw)