Once I set myself the challenge - I was on a train platform in Tokyo, and I heard some people talking near me, and I started listening just trying to place my best guess as to where they were from. It was pretty hard. Eventually I fastened onto one single language feature: these people were using glottal stop "t" (a "t" pronounced way back in the throat) in the middle of words instead of flap "t" (like in American "batter"). That one difference told me I was listening to British English instead of Australian English. The rest of it - vowels, intonation, everything else - was at the time too subtle for me to distinguish.
Why in the world are these accents so similar? It turns out that when Australia was first settled, starting in 1788, most of the people who moved there came from the same area of England where Cockney speakers live today. A lot of them were convicts. My Aussie husband will tell you that these folk were subjected to a trip to Australia for petty crimes, like stealing or poaching, rather than anything more serious. Who'd want to be stuck on a ship full of murderers for six months? But as a result, both Cockney and Australian English are actually "daughter languages" of the same parent, an English dialect spoken in a particular region (and by a particular social group) in London at the end of the 18th century.
It's been more than two hundred years since then, and at the sound level, the two dialects are remarkably similar. There are more noticeable divergences of vocabulary, of course (for example, Australians say "truck" instead of "lorry") but a lot still remains common (such as saying "lift" rather than "elevator").
I remarked in my earlier post on dialects that the longer a language exists in a particular area, and the more isolated regions are, the more dialects will diverge. In the United States, there are isolated regions in the East (such as Appalachia) which preserve language features that haven't been present in a standard American dialect for hundreds of years. These are in fact useful for scholars who study language change.
You probably already know how I'm going to be connecting this to speculative fiction. It becomes relevant in all kinds of contexts. One possible science fiction context is that of extrapolating the language used by future societies (I think immediately of Mike Flynn's The January Dancer and Up Jim River). One possible fantasy context is that of quoting ancient texts (I think of Tolkien). Either science fiction or fantasy can easily support the idea of two societies that have been isolated for a long period of time suddenly finding one another again and having to resume communication (I think of Stargate, and one of my own planned stories).
If you're writing a story that involves language change, it's useful to consider the following factors:
1. amount of time elapsed
2. presence or absence of written language (this can slow change)
3. amount of intercommunication between isolated groups (more communication can mean slower change)
4. amount of intermixing with other language groups (this can accelerate change)
It's also useful to consider that change can occur in any of the following features:
1. phonology (consonant, vowel systems, etc.)
2. morphology (verb conjugations, noun pluralization, negation, etc.)
3. vocabulary (some words lost, some words new)
4. syntax (probably not the main word order, like subject-verb-object for English, but phrasings can vary a lot)
5. discourse (the order in which thoughts are presented, for example)
6. politeness (all kinds of manners may change along with social activities)
When you think about the degree of change that you want in your language, here are some English-language landmarks that you may find useful.
Old English: Beowulf, dated variously from the 8th or 11th centuries, so between the years 700 and 1000
hwaet we garde na, in gerdagum, theod cyninge, thrym gefrunon, hu the athelingas ellen fremedon.
(I have at least one friend who is better conversant with the proper format of this line. This is my rough transcription of a portion I memorized solely by sound - somewhat improved by reference to internet sources.)
The words I know have remained most similar to modern English here are "in" and "hu" (who) and "the." I think "gerdagum" means "those days" which sounds a lot like German to me. Needless to say, not a lot is comprehensible after more than a thousand years.
Middle English: Chaucer's The Canterbury Tales, written between 1387 and 1400
Whan that Aprille, with his shoures soote
The droghte of March hath perced to the roote
And bathed every veyne in swich licour,
Of which vertu engendred is the flour...
Okay, this is much, much more comprehensible, but still pretty tough. Consider also that its pronunciation is quite well reflected in the spelling of the words, so that gh is actually pronounced like "ch" in the German "ich". In addition, "flour" is actually "flower." So here we've got a pretty serious degree of difficulty. Amount of time elapsed: 600+ years
Shakespeare's English: excerpt from "The Tempest," written 1610 or 1611
I have done nothing but in care of thee,
Of thee, my dear one, thee, my daughter, who
Art ignorant of what thou art, nought knowing
Of whence I am, nor that I am more better
Than Prospero, master of a full poor cell,
And thy no greater father.
This should be much more familiar to a general audience. And while it is written in verse, it does give us an indication of the kinds of phrasings and vocabulary used in this time period, because Shakespeare's plays were intended to be performed for the general public (I would argue that they still come across better read aloud than read silently). Time elapsed: 400 years.
A last couple of notes: slang is always present, and changes pretty rapidly, but may not always be incorporated into the main thrust of change in a language. Also, language does not always simplify, nor does it always complicate - it will generally simplify in some areas of the language and complicate in others.