I mean “But we should consider that bodies are [...] a mere appearance of who knows what unknown object; that motion is not the effect of this unknown cause, but merely the appearance of its influence on our senses; that consequently neither of these is something outside us, but both are merely representations in us” seems pretty unambiguous to me. Kant isn’t saying here that ‘we can only know stuff about mind-independent objects by using language and concepts and frameworks’ in this passage; he’s saying ‘we can only know stuff about mere representations inside of us’.
Kant passages oscillate between making sense under one of these interpretations or the other (or neither):
the “causality interpretation”, which says that things-in-themselves are objects that cause appearances, like a mind-independent object causes an experience in someone’s head. If noumena are the “true correlates” of phenomena, while phenomena are nothing but subjective experiences, then this implies that we really don’t know anything about the world outside our heads. You can try to squirm out of this interpretation by asserting that words like “empirical” and “world” should be redefined to refer to subjective experiences in our heads, but this is just playing with definitions.
the “identity interpretation”, which says that things-in-themselves are the same objects as phenomena, just construed differently.
Quoting Wood (66-67, 69-70):
Yet the two interpretations appear to yield very different (incompatible) answers to the following three questions:
1. Is an appearance the very same entity as a thing in itself? The causality interpretation says no, the identity interpretation says yes.
2. Are appearances caused by things in themselves? The causality interpretation says yes, the identity interpretation says no.
3. Do the bodies we cognize have an existence in themselves? The causality interpretation says no, the identity interpretation says yes.
[… N]o entity stands to itself in the relation of cause to effect. Transcendental idealism is no intelligible doctrine at all if it cannot give self-consistent answers to the above three questions. [...]
Kant occasionally tries to combine “causality interpretation” talk with “identity interpretation” talk. When he does, the result is simply nonsense and self-contradiction:
“I say that things as objects of our senses existing outside us are given, but we know nothing of what they may be in themselves, cognizing only their appearances, that is, the representations which they cause in us by affecting our senses. Consequently, I grant by all means that there are bodies outside us, that is, things which, though quite unknown to us as to what they are in themselves, we still cognize by the representations which their influence on our sensibility procures us, and which we call bodies, a term signifying merely the appearance of the thing which is unknown to us but not the less actual. (P 4:289)
The first sentence here says that objects of the senses are given to our cognition, but then denies that we cognize these objects, saying instead that we cognize an entirely different set of objects (different from the ones he has just said are given). The second sentence infers from this that there are bodies outside us, but proceeds to say that it is not these bodies (that is, the entities Kant has just introduced to us as ‘bodies’) that we call ‘bodies’, but rather bodies are a wholly different set of entities. Such Orwellian doubletalk seems to be the inevitable result of trying to combine the causality interpretation with the identity interpretation while supposing that they are just two ways of saying the same thing. [...]
Kant of course denies that we can ever have cognition of an object as it is in itself, because we can have no sensible intuition of it—as it is in itself. But he seems to regard it as entirely permissible and even inevitable that we should be able to think the phenomenal objects around us solely through pure concepts of the understanding, hence as they are in themselves. If I arrive at the concept of a chair in the corner first by cognizing it empirically and then by abstracting from those conditions of cognition, so that I think of it existing in itself outside those conditions, then it is obvious that I am thinking of the same object, not of two different objects. It is also clear that when I think of it the second way, I am thinking of it, and not of its cause (if it has one). From this point of view, the causality interpretation seems utterly unmotivated and even nonsensical.
The problem arises, however, because Kant also wants to arrive at the concept of a thing existing in itself in another way. He starts from the fact that our empirical cognition results from the affection of our sensibility by something outside us. This leads him to think that there must be a cause acting on our sensibility from outside, making it possible for us to intuit appearances, which are then conceived as the effects of this cause.
Of course it would be open to him to think of this for each case of sensible intuition as the appearance acting on our sensibility those a wholly empirical causality. But Kant apparently arrived at transcendental idealism in part by thinking of it as a revised version of the metaphysics of physical influence between substances that he derived from Crusius. Thus sensible intuition is sometimes thought of as the affection of our senses by an object not as an appearance but as a thing in itself, and transcendental idealism is thought of as having to claim (inconsistently) that we are to regard ourselves (as things in themselves) as being metaphysically influenced by things in themselves.
Such a metaphysics would of course be illegitimately transcendent by the standards of the Critique, but Kant unfortunately appears sometimes to think that transcendental idealism is committed to it, and many of his followers down to the present day seem addicted to the doctrine that appears to be stated in the letter of those texts that express that thought, despite the patent nonsense they involve from the critical point of view. The thing in itself is then taken to be this transcendent cause affecting our sensibility as a whole, and the appearance is seen as the ensemble of representations resulting from its activity on us.