How do I import taxonomy terms
As popular Drupal 7 modules like Taxonomy CSV import/export and Taxonomy Manager are not ported to Drupal 8, I recommend a very simple but functional module called term_csv_tree_import.

After installing, go to /csv-tree-import and paste your taxonomies into the form. It assigns parent values from right to left, empty intermediate elements are allowed. You have to convert your CSV into the following format:

"element","subelement","subsubelement"