These language packs can also be downloaded separately, then added to an installation through the DISM tool:
dism /online /add-package /packagepath:c:\temp\langpack\de-de\lp.cab
Once an installation has multiple languages installed, the language can be switched through a system tray applet. Another use-scenario could be when you want to have a single image and switch the language at installation time using the same DISM command. All the UI languages installed can be seen in the registry under key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\MUI\UILanguages.
Even though language packs for non-Enterprise and non-Ultimate SKUs can be installed and a single one used, this isn’t within the license rights and you should use a local version of Windows for the language you want to be installed. If you want a single image with many languages or to change the language, then you need the Enterprise or Ultimate editions that support MUI.