Add language pack even if the feature is missing / unavailable in Windows 7 (or Vista)

On computers purchased from manufacturers like Dell, HP..., a Windows 7 OEM version is installed.
In these OEM versions, contrary to Microsoft contracted versions, the button to add a language pack from a MUI is unavailable (Control Panel / Region and Language / Keyboards and Languages).

If you bought your system abroad or Windows doesn't match the user's language, you can still do it successfully on Windows 7 and Vista with this little utility : Vistalizator