Ooooh Doggy!

The show Firefly is set in the 26th century, about a crew renegade space pioneers. Think wild west meets Star Trek meets awesome. The crew of the ship all speak like cowboys, saying things like, "Y'all see the man hanging out of the spaceship with the really big gun? I'm not saying you weren't easy to find. [But] It was kinda out of our way, and he didn't want to come in the first place. Man's lookin' to kill some folk." 
Han Solo is similar. He's the character who says yeah when others say yes; who uses contractions when others don't. His language is loose and common where others' are exotic or technical.
In the London cast recording of Les Miserables, all of the poor trash characters sing with cockney accents; substituting f sounds for th sounds, dropping the ends off of words.
Tolkien, I may have mentioned before, makes all of the hobbits use words of Anglo-Saxon origins, Tolkien did it to make them seem earthier, commoner. The elves often use words with roots in the Romance languages. 
In none of the examples above would the characters actually be speaking English, or anything we know as English at any rate. It's neat how our brains can skip the obvious fact that ain't won't carry nearly the same baggage hundreds of years from now. There aren't any French folks out there speaking French with cockney accents. But our brains never think to make those connections, and so the vocabulary choices work; we don't even know it.