Added Korean translation

[KANGPUNGYUN]

Description

I translated Euclidean algorithm and Magic square explanation into Korean. And I added missing value in _magicsquare.md.

Checklist

• [x] A description of the changes in this PR is mentioned above.
• [x] All the new and existing checks pass.
• [x] No plagiarized, duplicated, or repetitive documentation that has been directly copied from another source.
• [x] If it's a new explanation, it contains solid, understandable, and accessible information.
• [x] I have read the whole contributing guidelines and agree to the Code of Conduct.

[KANGPUNGYUN]

You're right. Thanks! I'll fix it right away :)

[KANGPUNGYUN]

No file written in other languages.