If I create a German language pack using "Sie", can I then create a child language and only translate the phrases containing "Sie" and change them to "du"? (instead of creating two separate language packs with all phrases translated)
I believe if you have a master language, and you edit a phrase in a child language. But not all, that the rest will show in English.
This sounds like it therefor be possible in parent/child languages. Because if you make a Parent german language, and a child german language, if the child is customized, the non customized ones inherit from parent.