Menu translation tab how-to?
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.