The question is a very good one. However the answer is unconvincing to me. By this reasoning, should someone make a jquery compiler that blocks the access to the original javascript language, and perhaps fills in any resulting holes in fuctionality, then jQuery would be a language. But how much would have really changed? And also, should then someone implement this new JQuery within a javascript environment, we would be talking about an embedded language within another language. but isn't that what we have now? I think to the extent that a library brings in new syntactic styles, it becomes more and more similar to a language.
As an aside, this got me thinking about compilers that can compile multiple languages in the same source file.. a kind of linguistic pick-n-choose... has anything been done along those lines?