Wordsworth's Preface to the Lyrical Ballads (1800) is one of, if not the, most important statements of the Romantic Period poets about how poetry should be composed and what its subjects should be. As your question suggests, poetic diction and poetic composition are the two main components Wordsworth concerns himself with in the "Preface." The short answer to your question is that these two components--although different--are integral to Wordsworth's vision of what we now call the Romantic Period in British literature.
Wordsworth argues in the "Preface" that instead of using lofty subjects and people--as was typical of Neoclassical Poetry (up to 1770 or so)--the proper subjects of the new poetry are
incidents and situations from common life, and to relate them . . . in a selection of language really used by men, and, at the same time, to throw over them a certain colouring of the imagination. . . . (Par. 5, ll. 1-4)
In other words, Wordsworth argues that poetry should concern itself with everyday life as experienced by common man. Equally important, the language of poetry should be the kind of language most men use and understand. In this brief passage, Wordsworth states his belief that poetic composition should center on the reality of the common man and that poetic diction should reflect real speech. "Language really used by men" did not mean, however, the various dialects used throughout Britain; rather, it meant that language which everyone used and understood--a dialect that everyone had in common.
Wordsworth goes on to argue that he chooses "humble and rustic life" primarily because "our elementary feelings coexist in a state of greater simplicity. . . . the passions of men are incorporated with the beautiful and permanent forms of nature." In other words, by focusing poetry on "humble" men and real-life incidents, the poet can truly capture the feelings men have about their lives and, by extension, nature itself. Expressing the common man's feelings--his response to happiness and adversity and nature's role in his life--was Wordsworth's (and Samuel Taylor Coleridge's) primary goal in the Lyrical Ballads, and two of the main components in that expression were the proper subject of poetic composition and the appropriate speech to convey life's experiences.