“We're in an atmospheric breakfast diner around noon, full of bustling men, women, and children.”

Atmospheric is odd, if you want something to be atmospheric describe how. The ordering is something, it might be improved like this: The small rise of music adorns the clatter of silverware and murmurs of the diner’s lunch rush.

“There are two men sitting together, and they look around the same age”
Two men around the same age sit together. Description should follow or proceed introduction.

[general comment about thing]
You’re sentences are starting to sound German. Vary them in length, have some long and some short, from your starting writing:
“_{^{We're in an atmospheric breakfast diner around noon, full of bustling men, women, and children}}. _{^{There are two men sitting together, and they look around the same age}}. _{^{The first one has black ruffled hair and pale skin}}. _{^{He’s wearing some good quality clothes, but you can see they're cheap}}. _{^{The other man has more brown hair, green eyes, and tanner skin than the first man}}._{^{This guy has more fashionable clothes}}.
See how at the end, when you are describing the two, they fall into a German dudududu pattern? Best way to solves this is just to have the descriptions of the character combine into one sentence.
[/general comment about thing]
“you”
I’d avoid second person because my Journalism friends would whip me if I didn’t vocalize my disagreement with it. (Because you’re essentially creating the reader as a character which may or may not be true depending how much of a traditionalist you give).

“He looks like he knows himself well.“
Fair enough. Cool line, could be ambiguous.

“They are talking indistinctivly”
Don’t invent new words unless you really really want to. They’re gossiping/mindless chatter/speaking without purpose/etc.

“so let's move on.”
You don’t have to tell me, you’re the one writing the story. Instead, just have a paragraph marking a change in characters/setting/time, we’ll know what’s happening, you don’t have to tell us.

“They look stable.”
Most people do, when you say something the audience will assume they’ll usually think you’re lying to them, so if that’s what you’re going for, yay! If not, boo.

“and he looks about 7ish”
You’re their God, unless you want to be an unreliable narrator, say he is 7.¹

“But according to their body language, it's something not so good.“
Paint me a word picture, how are their body language communicating the badness of their argument.

“They are fashionally dressed, and the girls have already figured out the world of makeup. The boy looks strong too!“
How does the boy looking strong relate (too) to the girl’s make up?

“He looks as if he were a football player.”
He’s wearing a jersey with the school’s beaver catching a football. Describe don’t prescribe.

[sub]1[/sup]
“In the very corner of the diner is a boy with a book. He looks like he doesn't want to be talked to. He's reading what looks like a history book? I can't tell what he's reading.”
Just to clarify: the above quote is a character’s perspective, however the “7ish” year-old boy is more narrator voiced. Yes, you can have both, and yes writers think it’s annoying that you can have both.

“There are employees in the back making and working the register“
Are they actually making a register? Don’t use blank and blank is what’s happening unless you have an essay with a word/page requirement because writing blanks and blanks is padding.

“That was a lot of exposition; it was a lot. But now we can get to the good stuff. Let's eavesdrop into these guys' conversation.“
Two things, firstly what you did was not exposition it was description. And secondly,

I’m not a fan of the meandering, from place to place, steadily on description of something or someone, because it acts like this sentence in how it has no end nor purpose and it just kinda is and ends unremarkably.

On a serious note, I do use the snapshots (what you’re doing) in book essays because they’re kinda essay and lazy. It’s sorta the first thing which pops into your mind like putting your characters in a blank/black room, and should be avoid by how low effort it usually is to come up with the idea.

Oh! Also also, you give less of a snapshot and more of a gendering of people? Like saying there’s a man and a woman over there, and there’s two men over there, but with tiny descriptors to each of them. Describe action and don’t get into a pattern with simply naming genders and relationships.

I guess you can hide things in those type of descriptions, but they need to go unstated, like one woman was eating a pickle, dadada put something here. Her friend’s food came with a side of pickles (ie, don’t state specifically that one doesn’t like pickles and the other’s taking them from her, but have it be there).

“You need to go to the bathroom to fix it." The brown haired man said.”
No tag should be without what it’s tagging. Fix it,” the brown haired man said. (question marks are fine, also quotes should have their own section like this:
“Hi,” he said.
“No! Don’t?” Was the reply.

“He suddenly switched languages into a strange unheard one.”
Eeeh, maybe. I’d rather you describe it. His fluent English transitioned into a harsh and crackled language with yelps and growls unknown to any tongue.

“So Auri walked into the bathroom and brushed out his hair.”
This should be it’s own paragraph. Paragraphs go at the end of a transition between people, place, things, speakers, narrators, okay so people just like new paragraphs and change scares them.

“Auri made his way out so he could keep a low profile.”
Describe how: ducking, covering, prancing, anything.

[tangent]
You put so much effort into describing little scenes, but so little indescrjbkmg...
That was meant to be time describing your main character, but I think I’m having a stroke and my hand is cramping from all this phone writing so yeah.