Mill acknowledges that it is common enough to begin a treatise on logic by discussing terms and other matters of language that there isn't really a need to explain why he is going to start with a discussion of language, but he goes on to discuss it anyway.
Language is evidently, and by the admission of all philosophers, one of the principle instruments or helps of thought; and any imperfection in the instrument, or in the mode of employing it, is confessedly liable, still more than in almost any other art, to confuse and impede the process, and destroy all ground of confidence in the result. For a mind not previously versed in the meaning and right use of the various kinds of words, to attempt the study of methods of philosophizing, would be as if some one should attempt to become an astronomical observer, having never learned to adjust the focal distance of his optical instruments so as to see distinctly. (p. 19)
This remark from Mill is a very similar thought to one advanced by Tim Williamson in "Must Do Better":
Philosophers who refuse to bother about semantics, on the grounds that they want to study the non-linguistic world, not our talk about that world, resemble astronomers who refuse to bother about the theory of telescopes, on the grounds that they want to study the stars, not our observation of them. Such an attitude may be good enough for amateurs; applied to more advanced inquiries, it produces crude errors. Those metaphysicians who ignore language in order not to project it onto the world are the very ones most likely to fall into just that fallacy, because the validity of their reasoning depends on unexamined assumptions about the structure of the language in which they reason. (p. 9)
I find the telescope/microscope analogy interesting, and compelling. Note that neither Mill nor Williamson is embracing the view that questions about language are the primary target of inquiry; rather they both liken the importance of understanding how language works to the importance of knowing how to use your tools.
In the next section of Chapter 1, Mill explains, more or less, the basics of his view of propositions. We are told that "the answer to every question which it is possible to frame must be contained in a Proposition, or Assertion" and that "whatever can be an object of belief, or even of disbelief, must, when put into words, assume the form of a proposition" (p. 20).
Mill goes on to characterize a proposition as "discourse, in which something is affirmed or denied of something" (p. 21), and analyzes propositions as containing three parts (subject, predicate, and copula). Throughout this section, Mill seems to be describing what an Early Modern like Locke called "Verbal Propositions", insofar as they are "formed by putting together two names", and Mill tells us that propositions "consist of at least two names". Similarly, when we were earlier told that the answer to every question is "contained in a proposition", or that propositions are a certain type of "discourse", it is clear that Mill is taking propositions to be something linguistic or verbal. Mill's propositions diverge, importantly, from at least one major strand of use of the term "proposition" in contemporary philosophy, and this will be important to bear in mind.
Mill ends chapter 1 with an argument in favor of studying names before studying things, by appeal to the fact that language was shaped by many people:
In any enumeration and classification of Things, which does not set out from their names, no varieties of things will of course be comprehended but those recognised by the particular inquirer; and it will still remain to be established, by a subsequent examination of names, that the enumeration has omitted nothing which ought to have been included. But if we begin with names, and use them as our clue to the things, we bring at once before us all the distinctions which have been recognised, not by a single inquirer, but by all inquirers taken together. It doubtless may, and I believe it will be found, that mankind have multiplied the varieties unnecessarily, and have imagined distinctions among things, where there were only distinctions in the manner of naming them. But we are not entitled to assume this in the commencement. We must begin by recognising the distinctions made by ordinary language. If some of these appear, on a close examination, not to be fundamental, the enumeration of the different kinds of realities may be abridged accordingly. But to impose upon the facts in teh first instance the yoke of a theory, while the grounds of the theory are reserved for discussion in a subsequent stage, is not a course which a logician can reasonably adopt. (p. 22)
Here I think we see an interesting commitment on Mill's part to a sort of qualified attention to ordinary language. There is something like a very weak presumption that distinctions made by ordinary language are legitimate distinctions, at least to the extent that one has to show cause to disregard them, rather than having to show cause for attending to them. This, I think, falls far short of a commitment to anything like the subsequent movement of ordinary language philosophy, but it is worthwhile to note that Mill explicitly references ordinary language (and not, say, specifically the technical vocabularies of past scholars).