I consider it a different language if for no other reason than, if a programmer familiar only with Javascript looked at code written in Coffeescript, they would not understand it at all. And it can't be converted to Javascript without either considerable hand work, or via an automated tool.
But as long as the underlying model is the same, and as long as it’s a leaky abstraction, I don’t think of it as a separate language.
For example, if it were to be a separate language I would look for its OOP stuff to be implemented in the “engine” and not to be strongly related to the JavaScript prototype mechanism.
In general, langages don't tend to have multiple grammars (although spoke languages tend to be looser than written), and perhaps the unnaturalness of having a second grammar might undermine your position.
For example, the (in)famous equivalence between 10[p] and p[10] point to C being a kind of translator rather than an entirely new way to think about programs.