I've blogged before about international sales. Of course, language is an issue. To some extent, iBooks has an edge over Kindle in terms of international presence. I see iBooks sales from Australia, Canada, and the UK, as well as in the US-- the larger English-speaking countries, in other words. Amazon has a Kindle store for the UK, but that's the only one in an English-speaking nation besides the US. There are Kindle stores in Germany, Spain, Italy, and France, but those folks are mostly not looking for books in English, so that doesn't help me a lot. I really wish there was a Kindle store in Canada! I do not know what the hold-up is.
iBooks also sells in non-English-speaking countries, but I only know which countries if I get a sale. I know they sell in the Netherlands, because I did get a sale from there (my first sale listed in euros). I think the giveaway of Shades of Empire must have helped make it more visible, because I also now have a few sales from the Kindle stores in Germany and Italy as well as the UK. More euros!