You are right, those are confounding factors.

Though if you look at willingness of people to take the bets moving in both directions, you may be able to account for it. For example (ignoring bilinguals for simplicity), if in England most people don't speak French, so they are less willing to move to France, and in France most people don't speak english, so they are less willing to move to England, maybe the effect cancels out, if both are equally represented.

Though, the experiment seems overly elaborate. We can look at immigration rates, and costs of immigration people are willing pay.