Let's consider, for a moment, that you have a space cadet who speaks like he's from Elizabethan era England. Does this pose a problem for your story? Not, if he happens to be Shakespeare and he has been transported through time, into space and has decided to join a spaceship team to someday be a ship captain. Most likely, though, you've chosen the wrong dialect for your:
4. All of the above
(Shakespeare in Space)
Often, readers lose interest and authors lose credibility when a character misspeaks. Vocabulary is something that must be considered when traveling to other times. Age is also something to be considered, I've heard that a rather famous book series contains teenage characters that speak like 30 year olds - now I've heard of old souls, but you'll lose my interest if it's obvious that the character is saying something the 30 yr old author would say, not what a 17 yr old character would say. It's lazy writing.
So, to make your characters and worlds believable remember to keep the language believable.
As Shakespeare in space would say: "Keep it real, bawcocks*!"
*Bawcock - A fine fellow; - a term of endearment