One of the things I like seeing in a work in which most of the characters are coming from different places is an understanding that they’re going to have somewhat different dialects. That’s awesome; it makes it clear to me that the author/GM knows that people sound different, and can fake the distinctive features of one dialect or another. What isn’t quite so awesome is when they’re exaggerated to the point where I feel like I’m being hit over the head with them. Here are some tips for not overstressing a dialect.
Leave the phonetics at the door. It’s not that I think there should be a blanket ban on writing those sorts of phonetic accents, and I have in fact seen them done very well. On the other hand, they slow even a proficient reader down, and they’ll drive your spellcheck crazy. Doing them in voice is easy. Actually rendering every little quirk in text…. I wouldn’t. If you must, stick to particularly mispronunciation-prone words and other characters’ names. Speaking of which—though this happens less often—while it might be tempting to change which accepted spelling you’re using in dialogue as compared to in the narrative, don’t do that either. Indicating a British accent by having “colour” and “favourite” and so on pop up, but only in that character’s dialogue (or vice versa if your default setting already allows for those extra vowels, isn’t going to do anything but confuse your spellcheck. Text, sure. Voice? No.
Use words characteristic to the dialect, but only when they come naturally. I can think of at least three different ways of rendering “you-plural” that pop up in different US dialects, and sticking one in, along with a couple more subtle indicators of accent, will generally be enough for me to figure out what sort of dialect I’m reading. On the other hand, if someone’s so desperate to get across a Texan accent that we start seeing y’all for you-singular, particularly in places where they’re having to work at it to get even a singular “you” into dialogue in the first place, we have a problem. Same goes for slang; it’s much better to put it in only where it would reasonably occur than to find every opportunity possible to throw in really dialect-specific slang and idioms… at least, unless it’s being played for laughs in an already humorous context, in which case we can talk.
Try to stick to word patterns and things that bend the rules of speech you’re used to. Some dialects are much fonder of multiple negatives for emphasis than others are. Some eschew contractions, some wallow in them. You might have something that goes in for questions rather than statements to make a point, or someone who tends to dispense with certain definite articles. Then there are dialects in which verbs don’t conjugate quite the way you’re used to, and ones that will never use a genderless pronoun when a gendered pronoun can serve, and all sorts of other interesting traits of languages, real or imagined, that stand out when adapted to whatever language you’re writing in. Just remember moderation—you want these to provide a flavor, not to whomp your audience with a sledgehammer.
Dialect, in short, isn’t a few large things; it’s a mass of small factors working together.