It's not so much that the Norse learned French, it's that the Norse settlement in Normandy tended to be localized to certain places like Rouen, and on the whole did not displace the native Franks, who remained the majority. The Norse also intermarried with the native Franks almost immediately, so by the time you get to William the Conqueror's day the Norse had long since been absorbed by the Frankish majority. The Normans of William's day spoke a dialect of French, because their ancestors had also been Franks.