For the property of "og:locale," the plugin calls the WP function: get_locale().
Unfortunately and oddly WordPress uses "ja" instead of ja_JP, which many other web services including Facebook use. FB has been strict on the locale setting and returning an error.
I modified the line 181 with apply_filters like:
echo '<meta property="og:locale" content="' . strtolower( esc_attr( apply_filters('wpfbogp_locale', get_locale()) ) ) . '"/>' . "\n";
I can add "add_filter" to my own functions.php to return "ja_JP" in the plugin.
Please consider this modification for the next version so that we can update it without losing customization. (The option setting for locale would be nice as well)
Thank you for developing the plugin.