You can change the system code page using the Control Panel. The following instructions are for Windows 7 (Windows 8). The procedure is similar for the other Windows versions from Windows Vista and newer.
In Windows 7 the settings for non-Unicode also found a new place in the Windows settings. Change system locale can be found under the tab Administrative in the box Language for non-Unicode programs. The bad news: You still have to reboot to make this settings effective.
Start the Control Panel, and open Regional and Language Options. In Windows 7 the settings we are looking for are located on two tabs. On the Formats tab you find settings for date and time formats.
On the Location tab you set your current location.
In Windows 7 Ultimate there is an interesting new feature. You can change the display language of Windows. The good news. You only need to relogin to set the new language becomes active. The screenshot shows a system with installed English and German display language. Microsoft offers more than 30 display languages for download to Ultimate edition users.