A Luxembourg is approximately 1/5 of a Wales. No idea why the Guardian would be using Luxembourgs when the world has settled on the Wales as the unit of land area.
A double decker bus should only really be used for volume. You want football pitches here. One deciwales is around 323000 football pitches, so a Luxembourg is roughly 646000.
Etymologically, "Wales" is already plural. The root is "wealh", meaning "foreigner" (and before that, a tribe who lived in the region). "Cornwall" contains the same root.
Of course in this case "Wales" is its own word, independent of its etymological root. Since it's an English word, you pluralize it the same way you would any other word ending in s: "Waleses".
The same construction applies to the founder of Wikipedia with his family: The Waleses.