In fact the only noticeable difference is the title. In Spanish it's a rather generic 'Prince of Evil'.
I found out this week that there won't be a Spanish King of Thorns or Emperor of Thorns. I got paid my advances but Principe del Mal didn't sell well enough for the Spanish publisher to want to sink the extra cost of translation in, so they cut their losses.
Why did a trilogy that has sold over a million copies world wide and is still going strong sell not much more than 0.1% of that number in Spanish? National taste? Marketing? I really don't know.
What I do know is that it's a sobering fact. If I were Spanish my books may very well have flopped and I would have disappeared from the shelves with scarcely a whimper.
I've no deep insight to offer here, just questions to offer up. Did a butterfly beat its wings one way in Spain and another in the UK/US? Is success so ephemeral? Or does a road trip of a few hundred miles from London to Madrid really bring around some deep seated difference in reading preferences?
As a side note, in Brazil, Hungary, and France it's doing really well (under its original title, or a closely related one). I can't give a full list though as sales figures from overseas publishers are as rare as very rare things (*).
(*) I save the good analogies for books.
UPDATE: The Spanish rights for King and Emperor are mine now & my agent is asking for the early return of the rights to Prince (it will be mine in 2016 either way). At the same time my agent is seeking a new publisher for the trilogy in Spanish.