If you see <html lang="en-US" prefix="og: http://ogp.me/ns#"> in the context of your website and decided to change the lang because osf SEO reason there are different ways as below :

1- Create a global variable $locale in your wp-config.php:

Copy to Clipboard

2- Create a global variable $locale in your wp-config.php:

Copy to Clipboard

3- Filter locale:

Copy to Clipboard