In Windows Vista and 7 only the Ultimate and Enterprise versions have the feature which allows you to install a languagepack.
But in the other versions you can do the same thing using the console and regedit.

In order to install a new language (in this example de-DE and the languagepack is located in G:\langpacks\de-de\lp.cab) you have to do the following steps

1. Download the language pack in the language you want to install
2. Make sure that there is an lp.cab file inside the folder of the language pack
3. Open the Console (CMD) with administrator rights (Run as administrator)
4. Type the following commands (note that you have to set the correct path of your language pack)

   dism /online /add-package /packagepath:G:\langpacks\de-de\lp.cab
   bcdedit /set {current} locale de-DE
   bcdboot %WinDir% /l de-DE
   

5. Remove the registry folder HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\MUI\UILanguages\en-US (remove all folders except the one with signature of you language pack)
6. Reboot your computer

Now you should see the system in the language you installed.