fix `config.json` and add Korean translation

[chabulhwi]

• In the config.json file, the ISO code representing Korean should be ko, not kr.
• Remove the client/public/locales/kr subdirectory.
• Add the client/public/locales/ko subdirectory, which itself is the OmegaT project for the Korean translation of lean4game.

OmegaT[0] is a translation memory application intended for professional translators. I use it to translate English documentation into Korean.

[0] https://omegat.org/

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• [ ]: depends on: #267
• [ ]: depends on: #268

[joneugster]

Thank you for the contribution!