YaST up to now has been written in its own language, YCP, but for several months now has been in the process of being converted to the Ruby programming language. As of last week, the developers feel it will soon be time to push the translated code into openSUSE Factory for the 13.1 release, namely by the beginning of August.
As of last week, the installer is now working in the translated code-base with the YaST modules having been translated into Ruby and producing a working installed system. This is being done as the "YCP Killer" project by SUSE developers in Prague.
For more details on this new version of YaST, see this openSUSE forum thread and the linked mailing list post.