You can translate taxonomy slugs by enabling the Translate custom post and taxonomy base slugs option in WPML Settings.
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 checked.
You can now translate custom taxonomy base slugs just like taxonomy terms. To do this, use the WPML -> Taxonomy Translation page.
Yes, WPML can translate both custom taxonomy terms and the base slug.
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.