In stories, characters typically speak Merchant Pidgin, though they may use another language if all involved are sufficiently fluent. I'll render either as English with some words in the language in question, with a remark if they switch languages.
Where a future version of an extant language is spoken that the narrator doesn't understand1, I'll render it as its modern equivalent. Anglic appears as English, if as a dialect to distinguish it from the narration. It's closely related to Merchant, so most characters will understand some of it.
Creoles like Kentauran or Skavenja, and more distantly related languages such as Razdunig or Inghelsc, will be rendered as conlangs rather than modern natlangs. Ditto for languages without current relatives like Emesha and exolangs like Rav K'tai.
Language descriptions make heavy use of IPA[Wikipedia]. Please refer to the linked page if you're unfamiliar, there are explanations and sound samples..
1 Translation software may be commonplace, but few bother to download more language packs than they need for everyday life.
Many languages once spoken on Earth are still around, either due to their large numbers of speakers or by conscious effort. Some ancient languages have been resurrected, too. They all have evolved and diverged, adopting words and features from other languages along the way. Creoles are commonplace, especially as lingua francas. A few conlangs also have become naturalized.
Insults and Swear Words
Spacers swear like their proverbial seagoing cousins, and while 'fucking' is still a favorite intensifier, others are also widely used for flavor and variety, eg. accursed, blasted, bloody, blooming, cursed, damn, everloving, flaming, honkin', infernal, goddamn, living, motherfucking, or stinking.
Slurs are fairly rare and often on the mild side. The majority of insults aren't focused on a person's traits or origin, but rather on their behavior, be that cowardly, entitled, annoying or foolish. When somebeing gets very personal with their insults and targets another's appearance or the like, that means the relationship is beyond salvageable.
And of course there's the classics and their various permutations, eg. arseface, cockwaffle, dickbag, dipshit, fuckface, fucknugget, scumbucket, shitweasel, sleazebag, twatwaffle, or wankpuffin. A few slang words and more colorful creations are also used, see the glossary.