why does internationalization menu i18n_menu translate but never localizes?
As said in the 2nd choice Translate and Localize, there are two different ways for translating menus.

If you want to use the Menu items with language will allow translations option, set a link in each of your defined languages, but none in language neutral, and check in admin/structure/menu/manage/main-menu then in *admin/structure/menu/item/[ITEM_ID]/edit* for each set of translation, that it is correctly translated.

Now if you wish to work with the Menu items without language will be localized option,  make sure you create the menu item in the site default language (because localization always happens from the site's default language to other languages). And choose language neutral for the menu item when created. 

Now in the opened up Translate tab, you'll translate the menu item title and description instead of creating another menu item. And you're done !

For a complete explanation read this nice tutorial about Drupal multilingual navigation.