Override and set the Facebook open graph locale the Yoast WordPress SEO plugin puts in the WordPress head

add_filter('wpseo_locale', 'override_og_locale');
function override_og_locale($locale)
    return "en_GB";

The Yoast WordPress plugin adds the Facebook open graph markup to your WordPress head if you enable it. It takes the locale setting from whatever language you have set in the WordPress general settings. You can change it in the settings to whatever locale you need or, if you want to keep the WordPress language settings as they are, you can override it by putting this in your theme's functions.php