@Kokurokoki is making some really good points. I kind of want to argue with the language bit, but that'll distract from the other things so maybe later.
Like, end of the day, it's less authenticity of your dialogue than it is a certain way of thinking that will most likely make or break your representation. E.g.: I have only ever seen white writers suggest that there is someone so perfectly average that they can fit into any ethnic group with equal ease. This fantasty of the perfectly average individual is a quintessential conceit of those who have the luxury of not being Othered on a daily basis. As much as I might wish for genuine markers stating 'this dude is clearly from Honduras, or is Hmong or Zulu' I also acknowledge that I will probably miss 95% of your hard work (e.g.: It is very likely that no will notice or even be aware of the meaning of any of the translated Xhosa expressions I (once, so far) peppered into Wakandans' speech and those who do notice will prob say 'good try, but wrong'). What matters then, in a practical sense, is less positive representation (the presence of markers of authenticity) and more the absence of negative representation (markers of inauthenticity).
I doubt that it is possible you can truly have one without at least some of the other, but that's a different discussion.
I brought up RWBY's faunus and Season 1 Korra before as examples of horribly done fictional minorities. Yet, their storyline, taken without all the trappings, mirrors that of Erik Killmonger's in Black Panther who won critical acclaim and people genuinely felt conflicted about. An oppressed people try to instigate violent revolution to overturn the status quo on behalf of their oppressed brethern. Erik had significantly less opportunity to argue his case than did the non-benders and faunus, but why does this guy (who has to be admitted is at least a little cartoonishly villainous having killed 3000 people off screen and his girlfriend on) get all the praise and the fictional ones feel completely hollow?
Because in Black Panther Erik Killmonger is acknowledged as being right. Full stop, no ifs and buts, his methods may be disagreeable and will likely result in tragedy, but his anger and goals are acknowledged to be legitimate. "You were wrong! All of you were wrong!" T'Challa yells.
And when Erik dies, it is staying true to who he is: the child that dreamt of the Wakandan sunset, the most beautiful in the world, and the adult who would rather death than bondage, who demanded freedom at any price.
And that resonated with audiences.
The overly long fight scenes, the dragging pace of the first act, the oddly floaty CG, the Border Tribe so easily going over to his cause, the fact that the movie felt both rushed and long - all of that was forgiven because the story felt real.
You don't see this in Korra or RWBY - both have cartoonish caricatures as villains. One's actually a member of the oppressing group, the other's a creepy, violence-happy pedophile (or so I understand, I didn't watch too far into recent RWBY). By de-legitimizing the villain you de-legitimize their grievances, and while it's an easy out, it just smacks of a writer handling a topic beyond their ability.
If you bring up hard questions, you need to be prepared to answer them in a mature fashion. Offhand, fictional minority wise, only Scar from FMA really goes the distance in asking and answering those questions. Edward doesn't really acknowledge the legitimacy of his grievances, but the strength of the writing is such that you can tell this is a kid, and although smart, still part of the Armestrian majority. All the good guys in the army hew closer to Scar's view: what happened was an unforgivable sin and what they did cannot ever be forgiven.
Now, you may not be interested in handling such weighty themes. Sometimes a black dude is just a black dude.
But make sure that 'just' doesn't get either too stereotypical - aka, Chadwick Boseman apparently just gave a commencement address at Howard describing the first acting gig he got: a character with an absent father and drug-addicted mother and he was like 'this is woah stereotypical' and then was promptly fired - or too generic. Because 'generic' usually ends up as code for 'white' and although I hate to bring this up, privilege and power tend to be potent blinders. Like I said earlier, it's not that white people are perfectly generic, it's that certain cultural conceits have begun to dominate storytelling spaces and that dominance is assumed to be the default. Basically no one in Shanghai lives in a house - and yet it is still the kind of house kids will draw.
Writing minorities you need to be prepared to acknowledge differences in world view. And those differences can't simply be papered over: what informs their lives will be their lived experiences. If you cannot sympathize with many different kinds of people, that is the journey you as a writer need to be prepared to go on. In order to write Erik Killmonger you have to sympathize with Malcolm X, not the bad guy from your high school history textbook, the dark to King's light, but the man whom even his enemies described as possessing ironclad self-control, the man who preached black empowerment indepedent of white rule, the man who could even make the white cop responsible for wiretapping him go 'he kind of has a point?'
You can disagree with him, obviously! But if you consider his views illegitimate from the outset, you cannot write his story. It is a task that will be beyond you. Similarly, if you cannot symparthize with minorities because you don't know any, and you do not know their stories, then, once more, writing them is a task that you will fail at. This is the simple calculus of ignorance and one I think everyone can intuitively understand.
Edit: All this to say, this is where you
start. Be open to other ideas and other views. A writer is ultimately a thief, the broader your ability to understand, the greater your ability to write. So put in the time and do the research.
