Thank you, I will report this to the developer of Greenshift!
Meanwhile, I've run into another problem:
As you can see from the links I've already posted, the menu is structured to be accessed from a hamburger menu (also on desktop). This is implemented with the 'traditional' navigation block. However, when I try to translate the navigation items (using the FSE template translation description), WPML simply does not recognize the menu items (neither the titles nor the links).
What could be the problem here?