your form called https://example.com/en/cart
and ?
paramaters id_product & id_product_attribute & quantity_wanted ?
You need to pass parameters to ajax-cart, but you don't send them.
I wrote what the ajax cart needs.
If you think that using JS is bad, you are very wrong.
JS is faster and you have control over it.
The question is if you know JS. I don't mean it badly.
When you send via submit, you send data from forms.
If the recipient of the get / post does not know the parameters, it will do nothing !!!
I understand, you have your own solution, unfortunately I am unable to help you with it.
I'd like to help you, but it's really not possible to decode what you need.