Basic example of the hook for below operations.

1. Updating the user meta with the new locale using the update_user_meta() function.
2. Displaying a success message indicating the switched locale.

// Define a callback function to execute when the switch_locale action is triggered
function wpdocs_switch_locale_callback( $locale, $user_id ) {
    // Perform custom logic or tasks here based on the locale and user ID
    
    // Example: Update user meta with the new locale
    update_user_meta( $user_id, 'locale', $locale );
    
    // Example: Display a success message
    echo 'Locale switched to: ' . $locale;
}

// Hook the callback function to the switch_locale action
add_action( 'switch_locale', 'wpdocs_switch_locale_callback', 10, 2 );