Switch the language of the site based on a query param, ex: `site.com/wp-admin?l=en_GB`

language.php

<?php
/**
 * Plugin: Quick Langauge Switcher
 * Note, this only sets the site language, if your user has a user-specific language set, it won't change.
 */
// sv_SE, ja, ko_KR, ar, he_IL
function kdwan_switch_language( $locale ) {
	if ( isset( $_GET['l'] ) ) {
		return sanitize_key( $_GET['l'] );
	}

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