This thread is resolved. Here is a description of the problem and solution.
Unable to check the checkbox 'set language information for existing media'
It is OK that the option 'Set language information for existing media' is greyed out. It is like this in most of the cases, and it means that you do not need to take any action regarding this option and the existing media already has the language information. Please proceed with applying this option to the main site where it is not grayed out.
This is the technical support forum for WPML - the multilingual WordPress plugin.