"Do you want the United Kingdom to adopt the 'alternative vote' system instead of the current 'first past the post' system for electing Members of Parliament to the House of Commons?"
And, as it will appear on ballot papers in Wales:
"Ydych chi am i'r Deyrnas Unedig ddilyn y system 'pleidlais amgen' yn lle'r system gyfredol 'y cyntaf i'r felin' ar gyfer ethol Aelodau Seneddol i Dŷ'r Cyffredin?"
And, the answer you should place your X next to (in both cases): "Yes".