Menu translation tab how-to? I have a menu in the original language English. Spanish is active in admin/config/regional/language . Custom menu link is active in admin/config/regional/content-language . I now have a Translation tab in admin/structure/menu/manage/mymenu/translate. I click Add in the Spanish row, change the label and Description (only Description is sufficient). Then what?
How do I add menu items to the now supposedly created Spanish version? I tried several things but could not figure out where this new (empty?) Spanish menu is? It would be logical and consistent when now an "Add link" button would also appear in admin/structure/menu/manage/mymenu/translate/es/edit like in admin/structure/menu/manage/mymenu. 

What is missing is some simple help text right in the forms that guide our thoughts in the right direction. For instance what is the purpose of the "Menu language" select box? Is that to change the language of the whole menu? But wasn't the language fixed already at creation (the default language)? I could not find a decent explanation so I resort to the help of somebody who managed to got it working.
So far you've translated only the menu configuration consisting of Label and Description. The menu language select box is for the original language of the menu, in your case English.

The translation of menu links works independently by translating the menu links themselves, which is content translation and works the same as for nodes. You see a Translate tab when you click on Edit for a menu link in the main screen of the menu (after you've enabled content translation for custom menu links in admin/config/regional/content-language as you've already done).

The menu items are the same for all languages. If you want to have a different set of menu links for each language then you have to use separate menus.