In order to change the language in Microsoft Office you need to change the input language in Windows operating system. Open Start > Control Panel > Clock, Language, and Region > Change keyboards or other input methods > Change keyboards. Under Default input language you can select English from the list.
Click again Start and go to All Programs > Microsoft Office > Microsoft Office Tools > Microsoft Office Language Settings. In Microsoft Office Language Settings dialog box go to Editing Languages tab and check English language from the Primary editing language list.
For the additional information about the default language for Office programs you can go on Microsoft Office support page.