anyone knows where exactly the manufacturers (selection) sidebox forms its URL to send you to after making the selection?

there are two files:

1. includes/modules/sideboxes/manufacturer.php

but I think the URL is formed in:
2. /includes/templates/template_default/sideboxes/tpl_manufacturers_select.php

in that file I find:
$content.= zen_draw_form('manufacturers_form', zen_href_link(FILENAME_DEFAULT, '', 'NONSSL', false), 'get');
$content .= zen_draw_hidden_field('main_page', FILENAME_DEFAULT);

which I am pretty sure is the code I am looking for. Can anyone confirm?

And if so, how can I add the session language to this URL?
Now if I select a manufacturer the result always shows in the default page language...

Btw. this is URL that gets formed;
action="http://www.mysite.com/index.php?main_page=index"

Your help is really appreciated!