<?php
$date = DateTimeImmutable::createFromFormat('Y-m-d', '2023-09-23', new DateTimeZone('Europe/London'));

foreach (['en', 'xx'] as $locale)
{
    $formatter = new IntlDateFormatter(
        $locale,
        IntlDateFormatter::FULL,
        IntlDateFormatter::FULL,
        null,
        null,
        'w'
    );
    
    echo "Locale: $locale, Week number: " . $formatter->format($date) . "<br>\n";
}