"Dyna lle mae'r galon"
This translates literally as:
* Dyna: That is
* lle: where
* mae: is
* 'r galon: the heart
You could also say:
"Dyma lle mae'r galon"
This is a more formal way of saying the same thing, using "Dyma" (this is) instead of "Dyna" (that is).
Both phrases accurately convey the meaning of "is where the heart is" in Welsh.