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."

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Ar an cúigiú lá déag de mí Méan Fómhair, scríobh Ross Clark:

"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."

That part of the point of Lady Chatterley’s Lover, wasn’t it? That everyday rustic domestic speech was actually documented in the mouth of the love interest, and the associated class ructions with that.

--
‘As I sat looking up at the Guinness ad, I could never figure out /
How your man stayed up on the surfboard after fourteen pints of stout’
(C. Moore)

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