Which nid to use for translated node in a menu
In Drupal 7 you can use Locale core module to translate. 

Locale : Adds language handling functionality and enables the translation of the user interface to languages other than English


After enabling the Locale module , add German language using admin/config/regional/language.


Use English words to create menus.
After creating the menus use admin/config/regional/translate/translate link to translate English string to German.


Search for the string. I'm using About Us to translate. Click the corresponding edit link to translate.


Enter German string