What should a bilingual site look like from the backend?
I figured out that a correct localized main menu should be bilingual or multilingual and therefore look like this: 

Home
About Us
-Team
-Jobs
Start
Über uns
-Mitarbeiter
-Karriere
Infographic (EN: Infographic, DE: Infografik)

The possibility to save localizations inside of the node (my possibiliy 1 in the question) are for pages that are not necessary to translate (e.g. they show only a infographic, as in my example above) but the links pointing to them. 

Hope this helps other people with this question.