"It is her whose hat is yellow."
Translation:Det er hende hvis hat er gul.
"It is she", and "it is I" is correct English, but most people say "it is her" and "it is me". It reminds of a joke my English teacher used to tell: A man, who just passed away, knocked on the door of the pearly gates of heaven to be let in. St. Peter asks: "Who is it?". The man answers: "It is I", whereupon St. Peter says: "Ugh, another bloody schoolmaster!"