I have a Dutch language version of Windows 7 and downloaded English language packages for it, but there is no way to use them.
Do I have to install an English version of Windows 7 instead in order for them to work?
If you are using Windows 7 Ultimate, you can switch languages through the control panel. Otherwise you'll either have to upgrade to Ultimate edition (E.g. using Anytime Upgrade) or reinstall Windows 7 using an English version.