It is located in the language file. You can look for the following strings:

COM_EASYDISCUSS_POST_RESOLVED_TITLE="Mark resolve"
COM_EASYDISCUSS_POST_RESOLVED_DESC="Do you want to mark this thread as resolve?"
COM_EASYDISCUSS_BUTTON_YES_MARK_POST_RESOLVED="Resolve"
COM_EASYDISCUSS_POST_UNRESOLVED_TITLE="Mark unresolve"
COM_EASYDISCUSS_POST_UNRESOLVED_DESC="Do you want to mark this thread as unresolve?"
COM_EASYDISCUSS_BUTTON_YES_MARK_POST_UNRESOLVED="Unresolve"