Line 208 of the lingua.selector snippet has the following preg_replace to generate the other language url:

$pageURL = preg_replace('/^' . preg_quote($parseUrl['scheme'] . '://' . $parseUrl['host'] . '/' . $requestUri, '/') . '/i'
       , $parseUrl['scheme'] . '://' . $parseUrl['host'] . '/' . $baseUrl . $itemUri
       , $originPageUrl);

That's trying to find $parseUrl['scheme'] . '://' . $parseUrl['host'] . '/' . $requestUri in $originPageUrl, to replace it with the (correct) $parseUrl['scheme'] . '://' . $parseUrl['host'] . '/' . $baseUrl . $itemUri.

Because HTTPS is (transparently) served on port 443, the port is injected into $pageURL here in line 55 and therefore around line 175 into the subject of this replace, $originPageUrl.

Because $parseUrl['host'] does not have the port, the preg_replace fails and the user is given the current link with ?lang=foo added. While that works to switch the language, the user wants to see the proper localised url and not the current one.

By not adding the port if it's 443, the preg_replace matches as expected and the right URL is returned.