To change the interface language to English you'll need the English MUI (Multilingual User Interface)
The first thing to do is check to see if it's already installed, which you can do by checking the 'Region and Language' option in Control Panel. If its installed you can switch the language and reboot.
Unfortunately, if it's not installed it could be a problem as it's only available through Volume licensing or Select agreements, so check with the company you're working with.
Here's some details: