slang in the future
This same question came up on an 'Andromeda' mailing list not so long ago.
It raised a lot of good points but it boiled down to one very good answer from one of the writers assistants.
A: Everyone aboard Andromeda speaks Common, not English. It is translated not literally, but idiomatically (which means that while they might actually be saying a Vedran saying like, "Don't attack until you're within the sounds of their hooves", it would be translated to "Don't shoot till you see the whites of their eyes" which would be a more familiar saying to us) through your tv.
Many words and phrases we use today have their origins far back in time, sometimes 'really' far. So, why is it hard to believe that those words, and perhaps a few new ones (those from 'our' time), would make it into their century?
Hope this helps a little.