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.
Best Answer
As mentioned in the TechNet forum post that you linked and from my own personal experience, it appears that you need to have between 1GB and 2GB of RAM minimum for both Windows 2012 R2 and Windows 8.1 when using WDS.
I have just tried this on my test infrastructure with the following results:
The one piece that I am not sure about is why it seems to work sometimes when the language is set to some of the other options like Canadian (CN).