WPML is adding the language as a directory in pages/posts and in CPT you have selected to be translatable.
I suggest to remove the “Affiliate links” from the translatable CPT of WPML.
After that all your affiliate URLs should be yoursite.com/go/casino without the language in the URL.
Also if you need to create different affiliate links for each language for each casino you can create separate posts.
For example for Casino1 you can create a post “Casino1 link – EN” and “Casino1 link – DE” etc.