Expose user language through a DAV property

[tcitworld]

Required for nextcloud/calendar#3414

Needs to be added here

server/apps/dav/lib/Connector/Sabre/Principal.php

Lines 504 to 519 in 9585c4f

	protected function userToPrincipal($user) {
	$userId = $user->getUID();
	$displayName = $user->getDisplayName();
	$principal = [
	'uri' => $this->principalPrefix . '/' . $userId,
	'{DAV:}displayname' => is_null($displayName) ? $userId : $displayName,
	'{urn:ietf:params:xml:ns:caldav}calendar-user-type' => 'INDIVIDUAL',
	];
	$email = $user->getEMailAddress();
	if (!empty($email)) {
	$principal['{http://sabredav.org/ns}email-address'] = $email;
	}
	return $principal;
	}

And probably a few adjustments to make sure it's read-only.