If I use this Store link, which I had bookmarked years ago, it is in English and I can order. After ordering and clicking "Return to Store" I end up at the German Store and can't find anywhere to get back to the main English Store.
Does anyone have a link to the English Store Home page 
The store (store.guildwars.com) is temperamental. It's not uncommon to have issues with it. I typically have the opposite problem, with it being in German or some other language and then setting itself to English after navigating through a few pages. That's not happening to me now, though it is only displaying in basic html. Using different browsers tends to give different results so perhaps try that.
Locale settings on your browser or system?
