One of the things that's different here is that so far as I can tell, this world is entirely atheistic. There's a lot of math and magic and metaphysics, but no one has so far decided that any personal deities are attached to that stuff. (The math -- don't worry -- is implicit, not explicit. This fantasy novel does not come with equations.) (Metaphysics ditto.) So the obvious stuff to figure out is who does which social functions of religion and how and why, and most of that came to mind fairly quickly, and hurrah.
What is much less intuitive is the swearing, and yet it keeps coming up. Even the fake-swears: "darn it" might work backwards if you were applying it as a mild fake-swear akin to "patch it." But only if you'd set it up so that doing the mending was a source of much annoyance culturally, and why? Most of the other fake-swears refer to either religious or bodily concepts. Even my favorite Minnesota fake-swear, "Oh, for the love of Pete," is likely to come off wrong in a fantasy novel: "Oh, for the love of Ky'ythryinian'iel," is likely to give someone the impression that there is a deity or some other important figure of that hideous fantasy novel name, rather than that's a random person's name. And if it's, "Oh, for the love of [common name given to a spear-carrier in Chapter 4]," you may have readers wracking their brains for who that was again and why their love should matter.
"Oh, hell." Is none. "Damn it." See above re: cultural notion of hell. "My God, what were you thinking?" I was thinking that these people didn't seem to have one. At all. Nor do their neighbors. And you know what? I don't want their neighbors to. I don't want to set up a theist vs. atheist distinction in cultures in this world. This is sort of like when I look at a short story and have to put a boss character in and decide that the boss is female because my main character is female and I don't want to get into simplistic analyses of gender roles in that particular interaction. There are enough bits of cultural stuff to mess with here without making it look like the Evil Christians are trying to oppress the Good Atheists or vice versa. (See also Wicked Jews, Deeply Nasty Muslims, Awful Pagans, etc.: wish to dodge accidental statements about all real-world religions in this book, as that is not what I'm interested in here.) So there's no profanity borrowing, as atheists in our culture sometimes do, saying, "God damn it!" when they don't believe in God or gods or damnation or hell or anything related.
Then there are the vulgarities. This culture definitely still has shit, bullshit, ass, jackass, etc. It has genitals, to which I tend not to refer much as insults anyway, in part because those are supposed to be good bits, not insults. It has copulatory references from "screw it" on out through various permutations and levels of vulgarity to the average English-speaker's ear. But not all swearing is equal, and an English speaker would not snap, "Shit, Martha!" at their cousin in the same circumstances as they'd snap, "Jesus Christ, Martha!" So I can't just substitute vulgarity where profanity would go; it doesn't read right.
This is not an insurmountable problem, and I'm not saying it is. But made-up swear words are hard to get right. Red Dwarf's all purpose "smeg" works for several reasons: because it's the sort of noise an English-speaker could make in disgust, and also because Dave Lister's character is one who really would apply one word to every situation in his life, from a minor annoyance to grave peril, and having it be "smeg" is not that far off. Words like "frag" and "frak" and other transparent euphemisms for broadcast purposes sound like transparent euphemisms for broadcast purposes; they sound to me like adolescents who want to say "fuck" and not get grounded for it. Which is very different from having a culture in which no terms for sexual intercourse would ever be considered vulgar or offensive. So coming up with a word that people in this culture say when people in our culture would say, "Oh, God!", is not going to come out right; it's going to read like that word is just another gosh or golly, another spot where "God" originally went but didn't go this time for the sake of someone's sensibilities. The problem is that I don't want euphemism, I want a different outlook on the world completely. Euphemisms are easy. Shift in perspective are hard.
pameladean did a beautiful job with the swearing in The Dubious Hills, starting on the first page. I can't lift it, because doubt is a fact of life in this world and not an obscenity, but it worked for me; it didn't sound stilted but flowed and world-built and did all sorts of things you would want your swearing to do.
Working on it. But it turns up at odd moments. Probably when I'm two-thirds of the way through it'll start to feel natural, and then I'll be nearly done.