Open avast! settings, switch to "Language" page, click the link in the bottom ("To install additional languages, click here"), check English language / English help and click the "Next" button.
avast! downloads the English modules (provided you have an Internet connection, of course) and installs them; when you return to the "Language" window, you should be able to switch to English.
For the language change to take effect even on the resident protection windows, the computer should be restarted.