The idea of the language packs is precisely that: it transforms a certain user account to a certain language. All other user accounts use the default language.
My system has four languages installed. This computer I'm using now has an English Ultimate x64. My user account is in English, for my German wife's user account I applied German language pack, for my work purposes I also have user accounts in both languages used in my native Finland, Finnish and Swedish.
To change the language for a user account, first install language packs you need from Windows Update. Log in to account whose language you want to change
and go to Control Panel > Change display language
under Clock, Language and Region
(if you are using Control Panel Category View) or Control Panel > Region and Language
(if using Control Panel icon view). (1)
Open Keyboards and Languages
tab. Choose the language for this account from the drop down menu. (2)
Next, click Change keyboards
button. Add the keyboards needed and change the keyboard so it matches the chosen language. This is of course not necessary but normally users want the keyboard layout in their own language. (3)
Log out from this account, log in to your default admin account. Once again, go to the Region and Language
. Click now the Administrative
tab. Click Copy settings
button. Check both boxes under Copy your current settings to
This makes your computer to use your default language in logon screen, in all system accounts and in the new accounts you create.
Finally, a picture is worth thousand words. Here's my logon screen: