@traumflug Yep, the "No redirection" worked for @bzndk's suggestion. Thank you!
Generally speaking, though: Having shop.com/index.php?id_product=57 should be enough for the system to figure out what product is intended.
In my old shop software, you could give a URL like that and depending on the user's browser settings, it would show one or the the other language.
As the default URL is shop.com/index.php?idproduct=50&controller=product&idlang=2, it makes sense to be able to simply cut off the parts after id_product=50 and not rearrange the URL (thinking of copy and pasting URLs to different pages).