I’ve read Bishop Bruskewitz’s speech on “The Development of Doctrine”, but I’m looking for more quotes on the validity of the matter. Specifically, I’m looking for quotes (preferably from Magisterial documents) that say something to the effect of “our language can improve, but the core substance of the teachings can never change”.
Yeah, there is the quote from St. Vincent of Lerins, cited by Vatican Council I, at the end of chapter 4 on Faith and Reason:
"Hence, too, that meaning of the sacred dogmas is ever to be maintained which has once been declared by Holy mother Church, and there must never be any abandonment of this sense under the pretext or in the name of a more profound understanding. ‘May understanding, knowledge and wisdom increase as ages and centuries roll along, and greatly and vigorously flourish, in each and all, in the individual and the whole church: but this only in its own proper kind, that is to say, in the same doctrine, the same sense, and the same understanding.’ " (Vatican I, citing Vincent of Lerins, Commonitorium or The Notebooks, 28 [PL 50, 668]).
I think this quote sort of speaks to the OP’s point and yours.
“Then I have pointed out the truth, and shown the preaching of the Church, which the prophets proclaimed (as I have already demonstrated), but which Christ brought to perfection, and the apostles have handed down, from which the Church, receiving, and throughout all the world alone preserving them in their integrity, has transmitted them to her sons. Then also-having disposed of all questions which the heretics propose to us, and having explained the doctrine of the apostles, and clearly set forth many of those things which were said and done by the Lord in parables…that they may preserve steadfast the faith which they have received, guarded by the Church in its integrity, in order that they be in no way perverted by those who endeavor to teach them false doctrine…” Irenaeus, Against Heresies, Preface V (inter A.D. 180/199).