WPML 4.0 version introduced the possibility to translate custom taxonomy base slugs.
For example, let's say you have a custom taxonomy called "Book Genres", with some terms like "Science Fiction" and "Biographical". WPML allows you to translate all of them, including the base slug ("book-genres") of the "Book Genres" taxonomy itself.
To enable this, go to the WPML -> Settings page and scroll to the Slug translations section. Make sure the option to Translate custom post and taxonomy base slugs is enabled.
You can then translate custom taxonomy base slugs just like taxonomy terms. To do this, use the WPML -> Taxonomy Translation page.