Korean translated: index.js

Also editted some existing translations
Editted some korean: commits.js, branching.js

Existing translations were good, but I editted it though
Editted some Korean: sandbox.js

1. Editted existing Korean translations
2. Removed redundant comments
Translated into Korean: levelBuilder.js, nextLevel.js
Fixed mistranslation: nextLevel.js
Translated into Korean: confirmShowSolution.js
Translated strings.js into Korean: 18% done