I think we're getting into Sausurre again. See the ref below and note that even in English the French terms are used for the three categories of language: langue, langage, and parole.

The specific terms here, from google research (it's getting a bit beyond me at this level), are "meta-language" and "object-language" OR "language-object". (The first of these latter googles better than the second.)

So for the whole sentence:
Of monolingual dictionaries it is said that their meta-language contains the object-language identified with the language [then put "langue" in italics in round brackets] from which the headwords are taken.

You've got a tricky text there!