
How can the deaf guy speak japanese if he was born deaf? Sorry if I'm being stupid

I'm answering because I think you meant well.
Usually, deaf people who are able to speak are the ones who lost their hearing or they're not completely deaf (it's an spectrum)
BUT!
There are ways to learn how to pass air through your throat and create sound. Then, you learn how to shape those sounds, and a teacher coaches you about that, and teaches you how to speak. It demands a lot of effort, so some people born deaf don't always do it. And yes, they usually have a weird accent. They can't adjust their accent, because they can't hear it. So if they started learning to speak as a child, it's possible to give up because of bullying.
The deaf community is very protective and sometimes they don't bother reaching out to otsiders.
And also, each country has it's own sigh language. If you learn asl (american sign language), it's possible you won't be able to communicate with a person that only communicate with jsl (japanese one.)

Can someone reccomend me story like this with this trope where ml treats mc badly and doesnt care but then he regrets and falls in love with mc?

Yessss! Honestly that's like 50% of BL LOL. Not all of the ones I'm listing are at the same level of angst regarding how badly the ML treats the MC. Some are much more brutal, others fit the description a bit more loosely. These are just the ones I believe have a similar trope to what you're requesting that I've read or have angst.
Painter of the Night (completed but not all chapters are uploaded on here)
BJ Alex (completed)
Code Name Anastasia (on going, the love part hasn't kicked in yet so may want to let it marinate)
Jinx ( on going and again the ML love part hasn't been shown yet either)
Dark Fall (ongoing)
Down and Dirty (completed)
Legs that won't Walk (completed on side stories now)
Low Tide in Twilight (ongoing)
Passion (ongoing)
Profundis (ongoing)
Roses and Champagne (completed, side story ongoing I think??)
Yours to Claim (completed)
Limited Run (completed)
Crossing the Line (ongoing)
Paper Flower (completed)
The Third Ending (completed)

some of my other recommendations that I enjoyed reading are:
Dawn of the Dragon (ongoing)
Don't be Nice to me (completed)
On or Off (completed)
Love or Hate(Yeongha) (completed)
Dangerous Convenience Store (completed)
Caste Heaven (Yaoi) (completed ??)
Seeing Double (completed)
These don't follow the "a-hole ML with regret and angst" trope but I think they were good reads (▰˘◡˘▰)
Just finished rereading it and man I love them sm, throughout all the chapters I remembered my reactions when I first read it at every update with the fan translation. I miss those times