One of the biggest things new writers struggle with is listening to their characters' voices. To be fair, this is a challenge for everyone. I still struggle to analyze how characters talk well, especially in books. But if you don't learn how to listen to the speech patterns of your characters, each one's word choice will sound like yours. Which is boring - a character's unique way of talking separates their dialogue in the text from the prose, and creates a delightful reading experience.
There are several things you should consider when thinking about what a character sounds like. First off, where in the world are they from? What accent do they have? (Shadow Weaver is slightly Caribbean - fun fact.) If you listen to people who have the same accent as your character, you will begin to be able to hear the accent in your head. People from Britain phrase things different than Americans - think I have vs. I've got.
Another thing to think about is how educated your character is. If you read a lot, you'll have a bigger vocabulary. If you're more intelligent, you'll use bigger words. Even your parents' level of education can affect how you talk. You should also look at the tone the character talks in (loud, soft, deadpan, lilting, singsong, etc).
You can also use the good ol' powers of observation. Next time you watch your favorite show (shows are easiest to observe character voices in), pay attention to how one specific character talks. Think about their diction, their accent, their usual tone of voice. You can even mimic voices you heard from somewhere else in your books - I write Nell's diction to sound similar to the male coach on the Headspace app, although I think he has a deeper voice.
To practice, do those character monologues on "Backstory, Part 1: Advanced Interrogation". Have the character monologue for a long time about their childhood and backstory, letting the words flow from their mouth, until you can hear their voice and the words they'll use. Trust me, it'll come eventually, though it may take some practice. I didn't nail down Nell's voice until after the first draft of Starwalker.
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I also want to talk briefly about how to show dialectical oddities in your writing. Now, they don't really matter in your personal work - if you're writing a character monologue that no one will ever see, write the character's voice however the heck you want. But once you share your work with the world, writing accents and languages in a legible way is crucial. Let's discuss some ways to show these various differences well.
Accents
Sometimes you want to show a character's accent on the page to really show how they talk, especially if their accent is strong. The general principle to keep in mind is that words should still be readable if your character has a strong accent. A beta-reader (again, more on those later) will help you know whether your character's accent is too strong to flow well in the story. But sometimes it can be a way to accentuate just how strong a character's accent is. For instance, here's a passage from my monologues about Carmen:
I ain't got many friends in the Horde - they see me as an oddball, so I kinda stick to the quarters of my lady. I think she's the closest thing I can call to a best friend. I never shared a room with my lord - he found me a right annoying li'l pest, and dressed me in the cheapest rags he could find.
Other ways to show a strong accent are to mention it every so often, or have characters in the story ask the person to repeat themselves. As someone with a non-American accent, that will happen sometimes, especially if a character has other speech difficulties besides the accent.
Speech Impediments
The guiding principle here is to first, always do your research. Speech impediments or anomalies can be hard to live with, and you don't want to be insensitive. Aside from this, follow the principle of making sure the writing is legible. Mentioning the anomaly or having other characters ask for repetition can make a big difference.
Broken English
Take it from someone who taught herself Spanish: foreign language is hard. This is yet another area where you need to do your research. I think the easiest way to research a foreign language for a book is to actually dabble in one, and there are a lot of free resources out there. Duolingo and Memrise together make a great combo (I taught myself Spanish entirely through there, and used it to breeze through my school courses). Try taking two weeks' worth of lessons, just to get a taste of the grammar and feel of the language.
Usually, characters talking in a second language will stumble over their words, and pause for a bit to consider if they have a word in their vocabulary. They'll also make grammatical errors, usually in the areas their second language differs from their first. For example, I often find that knowing where to put the object pronoun in Spanish (Te amo vs. I love you) can be tricky! This is where research is important.
Sign Language
Full disclosure: I'm not deaf enough (yet) to need sign language. But doing some research and taking advice from other writers, sign language is actually not hard to write. I can't speak for BSL, but ASL is a very brief language. So it's up to you whether you want a very literal translation of the sign ("You deaf?" or "Are you deaf?"), or if you want a looser recounting. For the former, you'll probably need to do some research on the language. Most writers I've talked to about this advise italicizing the signed dialogue. You can also make note of the gestures if the character signing is angry or if you want to emphasize the dialogue. (You'll need to look up different signs for this to work.)
What are other language oddities you want to give advice on? Let me know! In the next chapter, I'll be discussing something I don't think gets enough press time: character flaws and arcs.