One of the things I've noticed a lot from writers (probably more of late than before this) is a general misunderstanding of how language works. Now I admit that I'm no linguist myself. If anything my writing is just as riddled with localisms that any writer's work is. However, I've learned through simple experience ways to counteract some of this to provide a richer, more realistic sounding story. Especially in the area of dialog.
So why is this so important? Well, believability for one. For example, if you have a British character, he's not going to run around shouting, "Awe, shucks, maw, I ain't going to no watering hole!" See my point? Sure, that's an extreme example, but you get the idea. That then brings me to my next point. There are two core voices in every story: The narrator (you) and the characters.
The narrator voice has a lot more flexibility in it since it's *your* voice. But it still needs to remain consistent (another problem I've seen in stories of late) and it needs to be executed with all proper spelling, word use and grammar, regardless of your chosen language. For example, one common failing with English writers is mixing up the words there, their, and they're or one I'm infamous for, led and lead.
The second voice, the characters, needs to follow a much stricter and more exacting set of rules. For example, with the British character, in our minds we might see them running around all day talking about "Tea and Crumpets". If your character is from the late 1800's, then yes, that's an acceptable element of their speech. If you're talking about a modern Brit, then no, as that terminology has been thrown by the wayside in the same way American English has evolved over the years. For example, we don't use the term "Score" as a reference to time, whereas those in the 1800's did.
So where does this all play into a writing exercise? Well, I'm getting to that. First I had to explain the two areas you, as a writer, were required to focus on. Since the narrator is you, all you need to concentrate on is cleaning up and ensuring that your spelling and grammar are in top notch shape, as you'll be graded by readers and critics alike on how well it's done.
As for the characters, they tend to give you a bit of a pass on that, but ONLY if you get the character elements right. So how do you go about doing that? Simple. Have you ever heard of people watching? It's really simple. Find a target individual or group of people who most closely match the personality and speech patterns of your character, and study them in depth for a period of time.
Usually this involves just sitting and listening to what they say, their body language, verbal grammar (including slang) and the content of the conversations. Now you don't necessarily need to head to your local mall and look like a creepy stalker listening in on what everyone says. You can find examples of your characters on TV, in other books, in your family, at your work, etc. IE, hunt around until you find the right person or persons you're looking to model your characters after.
Case in point, one of the baddies that I created for my novel "Lion in the Wind", a gentleman named "Ferrell Black", was modeled after Cardinal Richelieu in the 1993 movie "The Three Musketeers" played by Tim Curry. Now I didn't model the character after the actor, but rather the character he played, mostly because I needed a character with the ability to go from good to evil and back again with the flick of a switch, and the Cardinal allowed me to create the perfect mental picture on which to build and write Ferrell Black.
I've done this exact trick with nearly all of my characters, finding a core model on which to build my framework for the character, after which I use it to ensure that the character themselves is following that model as closely as possible to ensure consistency and accuracy. Having characters with rabid schizophrenia that are not supposed to have it is a very quick and effective way to drive away readers.
That's why it's best to effectively understand how people speak and how they interact with others, both those with compatible personalities and those with which they clash. And since you're working in the medium of the written word, language and its little innuendos carry far more weight with a reader than if they were seeing this in real life. So study the language, speech patterns, and other spoken elements of the person or people groups you wish to use in your story and learn them by heart. It'll improve your writing by leaps and bounds. :)