I am looking for an adjective to describe linguistic/symbolic capability in the most general sense, e.g. humans are x, rocks are not x. Certainly, there may be several words that fit the bill at greater and lesser levels of analysis, depending on what is meant by "language", and whether or not particular criteria of "capability" or "use" are being privileged. As I hope is clear in the above example, I would prefer a word that would be appropriate to designate the difference between a human and an inanimate object purely on the basis of linguistic capability. However, I am also open to using words that would be more at home in denoting more specialized differences in language ability, such as those which hold between different animal species, phyla, etc. — or even such differences as may be said to separate people with certain cognitive impairments from the general population — provided they could be used, at least metaphorically, to describe the general difference between those things that are capable of using language and those that are not.
Perhaps, verbal helps here. This comes out more clearly when we refer to the definition of non-verbal.
... humans are verbal, rocks (as well as animals in general) are not.
1 Relating to or in the form of words.
‘the root of the problem is visual rather than verbal’
‘Perhaps indirect rather than direct verbal strategies and nonverbal communication would be preferable in some relationships.’
‘The conversations between the bird beings sound as ‘bird brained’ as the rather mindless verbal dilly-dallying of the humans.’
Not involving or using words or speech.
‘Are you sending a non-verbal message that supports your words?’
Examples on the web:
articulate means able to speak. I suppose that's the word you're looking for: "humans are articulate"
However, "articulate" can also mean more than just the ability to speak, "if you describe someone as articulate, you mean that they are able to express their thoughts and ideas easily and well." Collins