When I log into iTunes store all settings are based on the United Kingdom, billing address, card details etc and the store language appears in English. When I open the App Store the UI is in German ...
After upgrading to iOS 5 my AppStore language changed to German. Referring to that quesiton it is possible to change the language by selecting the country in iTunes like it is described in this apple ...
I changed my iTunes account location to German (with an Austrian credit card) in order to download a certain app that wasn't available in Danish (my primary language). I was now able to get the app ...