I didn't downvote him, but one alternative is that a language is defined by a grammar. Of course you'd want to group certain very similar grammars together as one language (Python 2.x and 3.x), but the grammars of CoffeeScript and JavaScript are significantly different.

That’s also a reasonable definition. Since I didn’t say “if and only if,” both definitions are compatible. By the grammar definition, we could make a language with pure JavaScript semantics but Lisp-ish s-exprs, and it would be a different language as well.