Some default WordPress locales are not valid as Open Graph locales

By default, enabling Open Graph tags on our websites produces Open Graph validation errors.

This is because we have our multisite platform set with the “English (Australia)” locale, which is en_AU. Facebook only has en_UK and en_US.

It would be useful if you had some default mappings for non-existent locales (e.g. en_CA, en_AU, etc) to a close equivalent (UK or US), or displayed a notice that the selected locale was not available.