Wait to see how the Arduino web team will respond to your proposal. If they accept it then they will create a new repository on GitHub: arduino/reference-kr. This repository will initially contain a copy of arduino/reference-en, meaning all the pages will be in English.
Fork arduino/reference-kr. This will give you your own copy of the repository to edit (sebuls/reference-kr).
Clone your fork of arduino/reference-kr. This will put a copy of the repository on your computer. This step is actually optional as GitHub does allow you to edit from the website but it restricts you to only editing a single file per commit, which can sometimes be inconvenient, and you may prefer to work with the files with the software you have on your own computer, especially a decent text editor.
Create a branch in your repository to work on.
Edit the files with your translations.
Commit the edits you have made.
If you are working on a local clone, then push your commits to the remote. If you're editing directly from GitHub then this step is not necessary.
Submit a pull request to the arduino/reference-kr repository from the branch that contains your translations.
The Arduino team will review your proposed edits and may comment with requests for you to make changes to them, if so you will need to make the required changes and then commit (and push if working locally) them to the branch you submitted your pull request from, they will automatically be added to the pull request.
A member of the Arduino team will merge your pull request. Your commits are now in the arduino/reference-kr repository.
Once the translations of all the content is complete the Arduino web team will generate the arduino.cc web site content from the arduino/reference-kr repository and publish it to the web site.