Revert Mongolian translation into the correct one

[leregdos]

🐞 Problem
For some reason, between commits 249a94c and 61f8b92, the correct Mongolian translation I merged in #100348 has been replaced by a machine translated version.

🎯 Goal
If we successfully revert the translation into the correct and natural-sounding version, Mongolian users will get to understand the project more.

📋 Steps to solve the problem

1. Fork this repo.
2. Clone your forked repo into your local machine.
3. Open a new branch.
4. Copy the contents of README.mn.md from Add mongolian translation #100348.
5. Make a commit in your local branch.
6. Push your local branch and open a new PR into this repository.