For example, when writing, one tries to use proper grammar. However…
When I talk, I totes use a lot of slang. On paper, I’m legit. In reality, not so much.
So just listen. Listen to the local slang, accent, and other colloquialisms. Research the languages that your characters use if you are not familiar with them. Talk to people who do use the dialect you want.
Now, there are a few tips and tricks that can help you out in a pinch. These can help establish:
The younger your character is, the more likely it is that their words will slur or simply be incorrect. Infants babble. Toddlers commonly switch Ss with θs, Rs with Ws, and Ds with Bs. As they age and learn new words, they mispronounce or misuse them. Likewise, the elderly begin to lean towards raspy wording with lots of “sh” sounds due to tooth loss and wearing.
Much like with age, the level of education that a character has will impact the words they use. Someone without college experience might not know what a dariole is, but someone who went to culinary school might. In the same way, someone who talks about “furunkles” may have medical training. Mention of a labarum may suggest a background in history. Use of the word “tetradic” may lead one to think of artistic training. Know the lingo your character would use. Also, the more professional a person acts, the less likely they are to use contractions or slang.
This one is tricky. You don’t want to mock people, after all, but someone who talks about “sticking it to the man” is probably not caucasian. Heritage in different countries could result in phrases or words from that culture working into their speech. For example, ‘Grandfather” is Japanese, and the boy translating is first-generation Japanese-American.
“This is… Boooooring,” Griffen said. “You guys are boring. Do more kung fu!”
“Subete ga tatakaide wa arimasen.”
“Grandfather says, ‘Not everything is a fight.’ Also, that is seriously racist.”
Because of his heritage, he is equally fluent in Japanese and English. Furthermore, if you know anything about kung fu, you know why this is racist.