I've a WDS server running Windows 2012 Server and I just uploaded Windows 2012 R2 images to the WDS server.
Changed the boot.wim image to the one in the 2012 R2 ISO, but after this change, I was unable to select, for example, English (US) as default language. An error message appears with the following text:
Failed to set the selected language as UI language.
I can continue the installation with other language, but this is an annoyance. Another test was done rolling back the old boot.wim image from WS2012 but with this image I was unable to install WS2012 R2.
Some folks appears to be with the same issue as described here: http://social.technet.microsoft.com/Forums/windowsserver/en-US/681149bd-a204-48d4-ae93-557c1b950bfc/failed-to-set-teh-selected-language-as-ui-language?forum=winserversetup
The same problem happens if I take a boot image from Windows 8.1 for example.