Adding a “choose your language” landing page (customised index.php file)

If you go here: https://ganymedes-lgbt.nl , you see that I created a ‘choose your language’ page and replaced the index.php page (in public_html) with it – notice that the links don’t work. If I maintain the original index.php page, and keep this choose-your-language landing page as say, index_b.php, then the links in the landing page work. It doesn’t help if I copy the php from the orignal file to my custom file. How do I enable the links to function on a custom index.php file?