Force fr-CH language attribute on the HTML tag. Made for Jeff Legras (question asked in WordPress Academy Facebook group)

jba-force-language-attribute.php

<?php
/**
 * Plugin Name: Filter language attribute to force fr-CH.
 * Author: Jb Audras
 * Author URI: https://jeanbaptisteaudras.com
 * License: GPLv2 or later
 * Version: 1.0.0
 */

function jba_language_attributes() {
	return 'lang="fr-CH"';
}
add_filter( 'language_attributes', 'jba_language_attributes' );