How do the commonplace details of and the folksy language contribute to the impact of "The Lottery"?
I think this makes the impact surprising and increasingly strong.
First, it is surprising because we all get used to the regular suburban lifestyle and expect nothing out of the ordinary to happen among such folks. During The Lottery, we get this regular suburban feel for the majority of the story. It is not until the very end that we get the vulgar impression that something is wrong.
Second, when surprise is effective, and when an author can present life believably as exactly opposite from what it is, great impact can happen in literature. My first experience with The Lottery was on stage. I was horrified. I believed in the people of the town and in an instant, that was shattered.
To me, this is because they make the general setting of story seem so common and normal. This heightens the horror when we find out what is really going on.
I think that the main message of this story is that evil things can be done by normal people and in normal places. All that is necessary is for everyone to agree -- for them to think that it is in some way normal to do evil. The details and language that you mention emphasize this point. They make us feel that the people of the village are nice, friendly, normal people that we would like. That way, it hits us harder when they turn out to be doing such a horrible thing.