Seeing the source code it seems the language switcher is customized by the theme or custom coded and hence it is missing the correct link to the translated pages. Can you please clarify whether any customization added to the language switcher if so can you please point to us the exact code or location where the custom code is added.
I would also request temporary access to your site. it will enable me to check for a few parameters on your site dashboard. I have enabled private fields for you to securely share the details.