Here, it's worth noting David Drake's comments on translating from future/ancient languages:
One of the problems when you’re writing of either the past or the future is ‘How much should I translate?’ I don’t mean simply language: there’s a whole complex of things that people within any society take for granted but which vary between societies. (But language too: I had somebody complain that the Arthurian soldiers in The Dragon Lord talked like modern soldiers. My reaction to this was that I could write the soldiers’ dialogue in Latin, but the complainant couldn’t read it; and if I’m going to translate into English, why on Earth wouldn’t I translate into the type of English the same sort of men speak today?)
The desire for verisimilitude is nice, but excepting one part of language for "updating" while leaving the rest the same provides not an updating but a fraking bowdlerisation. Given that you're not going to be getting Ph.Ds in linguistics, don't bother updating your vocabularies. The cognitive dissonance engendered by one language's profanity, blasphemy, and scataological language which provides for the necessary frission of swearing in that culture's listeners doesn't translate at all well.
If you want a sense of the alien, change how florid different cultures are with their swearing. Floridity maps well to common English, but gives a sense of cultural distinction. If a culture swears in paragraphs/poetry, and the other grunts out single-syllable words, both are understandable, and different.
