Wordsworth promises to finish a Preface to the Lyrical Ballads (15/9/1800)

Well, yes, we have a letter of that date in which he promises his publisher that the Preface will be sent "in four days at furthest".
He had started writing it a couple of days earlier. The 15th was a Monday, so he means (says Crystal) "by the end of the week". Yes, very plausible, I've probably done the same. But he didn't actually finish it (we know from Dorothy Wordsworth's journal) until the 30th. Nevertheless, the book (poems by Wordsworth and Coleridge) was published, the following year, and the Preface is "now seen as a manifesto of the Romantic movement" (Crystal).
And the language link? Well, he says in the Preface that they are going to "choose incidents and situations from common life" and describe them "in a selection of language really used by men", whilst also doing something poetic with them. He also opines that in "humble and rustic life...the essential passions of the heart...speak a plainer and more emphatic language."
"Really used by men"?
"The language was certainly a great deal 'plainer' than the crafted elegance of many previous writers, but it was still some way from everyday rustic domestic speech, as pointed out by Coleridge in his _Biographia Literaria_ a few years later."