fix config.json and add Korean translation

* 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/

client/src/config.json

@@ -28,7 +28,7 @@
       "name": "Español"
     },
     {
-      "iso": "kr",
+      "iso": "ko",
       "flag": "KR",
       "name": "한국어"
     }