How can I add more alternate links for current page?

You can use 'bwsplgns_mltlngg_add_alt_links' filter to do that. For example, you want that search engines firstly indexed exactly English-language pages of your site for people from New Zealand and South Africa. Just add the following code into "functions.php" of your theme:

function get_additional_links( $links_attr ) {
/* English for New Zealand */
$links_attr[] = array( 'hreflang' => 'en_NZ',
'link_param' => 'en_GB' );
/* English for South Africa */
$links_attr[] = array( 'hreflang' => 'en_ZA',
'link_param' => 'en_GB' );
return $links_attr;
}
add_filter( 'bwsplgns_mltlngg_add_alt_links',
'get_additional_links' );

Note: before you have to choose English (for England) as one of languages of your website.
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