og:locale:alternate -> $lang->locale(LC_ALL)

[wdebusschere]

I think the og:local:alternate should be the full locale

<meta content="cs" property="og:locale:alternate">
<meta content="nl" property="og:locale:alternate">
<meta content="en_US" property="og:locale">

=>

<meta content="cs_CZ" property="og:locale:alternate">
 <meta content="nl_NL" property="og:locale:alternate">
<meta content="en_US" property="og:locale">

Code

if ($lang !== kirby()->language()) {
    $meta['og:locale:alternate'][] = fn () => $lang->code();
}

=>

if ($lang !== kirby()->language()) {
    $meta['og:locale:alternate'][] = fn () => $lang->locale(LC_ALL);
}