First of all, thank you very much for your patience and sorry to have you waiting - there is a very full ticket queue at the moment and it takes us a bit more than usual to answer tickets.
It's called "Ninja Forms Multilingual" and you should be able to download it from here after you log in with your WPML.org account: https://wpml.org/account/downloads/
After you install the plugin and activate it, when you go back to WPML > TM, you should be able to see the ninja forms inside that dropdown, too.
My issue is resolved now. Thank you!
It turned out that I had installed the WPML before I installed ninja forms..
After removing the WPML-ninja plugin and installing it again it worked