Hi, as far I understand open graph needs a lower case code for locale such as "en_us" or "el_gr" etc. WordPress uses a slightly different version like "en_US" with capital letters. That throws a warning in facebook debugger.
Simply adding a php function to make all country codes lower cases should fix the issue:
echo "<meta property='og:locale' content='".esc_attr( apply_filters( 'wpseo_locale', strtolower( get_locale() ) ) )."'/>\n";