How do aliens swear?
Not just aliens but all speakers of imaginary languages. Think about it. A polytheist who says “My god!” means something quite different than does a monotheist—and wouldn’t capitalize.
In Listening at the Gate I needed an epithet for use by Nondany, the itinerant master folklore collector. I settled on “By life!” I like it so much I want to get a slang wave going.
Recently a librarian nailed me with a steel eye and asked whether there was any “bad language” in the novel. I explained that it took place in an imaginary culture, but since all cultures have profanity, I’d had to invent some. She looked baffled. And bought the book.
WORLD-BUILDING and WILDERNESS: 
In the Paleolithic of Science-fiction, E.E.Smith PhD used invented slang in an attempt to provide versimilitude to otherwise bald and unconvincing narratives. His main contribution was substituting “QX” for “OK.” Much later, Linebarger, writing as Cordwainer Smith, used language in complex experimental ways to induce a sense of the strange and included slang in the form of idioms, proverbs, and rhymes – with varying success. The peril of this ploy is self-indulgence. Your switch from apostrophied to written-out is elegant and unobtrusive. U. K. LeGuin gives the “civilian” inhabitants of Roke Island the habit of replying to questions with cryptic evasions – a kind of stress-relieving response to the presence of so many student wizards and a play with the apparent tendency of peasants to use proverbs and set forms of speech the world over. “Morning is wiser than evening.”
Ah! Now I understand what the mallards that fly over the Rio Grande are shouting. “QX! QX!”