Having studied Locke's epistemological writing (as well as his political and ethical philosophies which for the sake of this discussion should be considered separate) in Philosophy of Language, I feel that Locke exposes a very important consideration for students of rhetoric, that language is the factor which introduces the highest degree of ambiguity to discourse and not competing theories of knowledge. Though I do think Locke utilizes strange lines of reasoning at times, I feel that he is entirely correct to assert that knowledge is non-relativistic, unambiguous, and clear whereas language, our tool of communication is steeped in arbitrariness and convention to the point where it obscures our own impressions of awareness, preventing them from being understood by others. Though the broadness of common language may make it sufficient for civil use (Locke says it doesn't, I'm really not certain whether or not that is the case yet), its lack of precision becomes particularly apparent in philosophical discourse. Simply think about how hard it is to establish non-problematic definitions of the most foundational concepts for this class such as "agency," "audience," or "work." Is it the case that we only have a redundant or tautological understanding of these concepts? Or is it instead the case that language fails us because every word has different connotations to every individual and their lived experience?
The relationship that words have to reality appears to be much less direct than the relationship ideas or sensations have to reality.
The concept of authority also enters Locke's discussion of language in that the apparent weakness of language can be attributed at least in part to the lack of any authority to enforce unified language practices. Words are further separated in the sense that they refer to the ideas of things and not things themselves. When hearing of pain, one doesn't actually experience pain themselves, but their understanding of the concept is stimulated and appealed to. Locke also notes that moral language seems to occupy a unique status in the sense that the foundational moral terms such as "good" are not derived directly from sense experience, but rather from setting aside a "blank epistemic slate" (can't think of a good term") for those particular sounds which is then developed through other ideas. Though it does appear to be true that language is necessary in some form or another for civil society to flourish, I think that Locke is right to suggest that language itself is the "weak link" which often complicates mutual understanding between individuals.