I just discovered that the Attributes section’s values are shared among a menu item for all of its translations.
Path in CMS: Home | Administration | Structure | Menus | Item | 59 | Edit menu | link
I started with English for a menu item. If I try to make the EN have the Class value of “abcdef”, it then shows that class for ES. If I remove it from ES, it disappears from EN. I need it to be different for the two translations of the one menu item.