servnest/http-errors/lib.php

<?php

$locale = 'en';
foreach (explode(',', preg_replace('/[A-Z0-9]|q=|;|-|\./', '', $_SERVER['HTTP_ACCEPT_LANGUAGE'] ?? '') as $client_locale)) {
	if (in_array($client_locale, ['en', 'fr'], true)) {
		$locale = $client_locale;
		break;
	}
}
header('Content-Language: ' . $locale);