I live in HK and I don't know German. But I find that when I click into iTunes store, I can guess the application but I don't know how to download since they are written in German. So how can I change the language into English?
If the language in the App Store is in German, you're in the wrong country's store. Scroll down to the bottom of the main iTunes Store page and where you see the country name and the round flag icon, pull that menu down and choose "Hong Kong".