How do I translate month names in post metadata?

One easy solution, if you want the frontend to be a different language than the backend, is to change the site language under Settings > General (Spanish for example) and then change your user account’s language by going to Users > Your Profile and changing the Language setting to English.

In addition, you can add this code to functions.php to make sure the language is English for you (or any other administrator) on the frontend. You could tweak the logic if you don’t want it to apply to all administrators.

/**
 * Sets the locale to English on the frontend if the current user is an administrator.
 *
 * @param string $locale The locale ID.
 *
 * @return string
 */
function sx372871_set_administrator_locale( $locale ) {
    // If the current user is an admin, make sure locale is set to English.
    if ( current_user_can('administrator') && ! is_admin()  ) {
        return 'en_US';
    }

    return $locale;
}
add_filter( 'locale', 'sx372871_set_administrator_locale' );