1. Is Test stand is completely new programming language than LabVIEW?
TestStand is not a programming language. It is a test executive.
It is more like Excel in that it implements expressions and conditional logic.
TestStand is combined with a language like LabVIEW, CVI, HTBasic etc... to create a test solution.
These languages are invoked as steps by use of an TestStand adapter.
http://zone.ni.com/reference/en-XX/help/370052H-01/tsref/infotopics/adapters/
It is also possible to use a script with an executable such as Perl, Python or TCL to perform tests.
2. How to convert Test stand program to LabVIEW and LabVIEW to test stand back?
Since TestStand is not really a programming language, there is no direct/simple way to convert to/from LabVIEW.