Say I choose to implement an interpreter or compiler for an existing programming language whose (only) reference implementation is distributed under the GPL. If in doing so I do not derive any code from this reference implementation and figure out the languages grammar/rules etc. from its official reference manual alone, does my interpreter/compiler still classify as a derivative work under the GPL? I.e. can a language "itself" fall under the GPL independent of any concrete implementation?
I'll answer these in reverse order because the second one is easier:
can a language "itself" fall under the GPL independent of any concrete implementation?
No. The GPL derives its legal power from copyright law, and copyright covers only the expression of an idea. The reference specification of a language can certainly be under the GPL (although the GPL is really designed for code, not documentation), but a language itself cannot be.
does my interpreter/compiler still classify as a derivative work under the GPL?
From your description, probably not, but what you're asking here is actually much more a legal question ("what does the law in jurisdiction X consider as a derivative work?") rather than an GPL question - the GPL does not (and cannot) define what a derivative work is.