Defaulting to a particular language is not what you should strive for. Suppose a japanese user living in France and fluently speaking French has his phone set to Japanese. Your App would default to German, despite it having a French translation; I guess that’s not what you want and it’s certainly not what the user wants.
For this reason I suggest to respect the language prioritization as set by the user.
Since you don’t want to support English but have developed the App in English, the easiest thing you can do is to just get rid of en.lproj right after compiling. This will leave the app with only the languages you plan to support, and the iPhone will chose the language best suited for the user, as set in iPhone’s defaults.
There is a pretty straightforward solution to deleting a specific localization:
Let Xcode build the App with all the present localizations and just before code-signing kicks in delete the en.lproj folder and all localized files for that language are gone. You can easily do this by adding a Run Script build phase to the target, containing this one line of code (it’s a Bash Script):
rm -r "${TARGET_BUILD_DIR}/${PRODUCT_NAME}.app/en.lproj"
Code Signing always kicks in after all build phases have completed, so just put that build phase at the end of your current phases.
Note that this increases the time to build because Xcode recreates all English files, so you might want to disable this step during testing.
I hope that’s a viable solution for you.
Concerning “the app must fall back to German in the case that an appropriate localization doesn’t exist”:
Question is what is an appropriate localization? If the user’s third choice is French (after 2 unsupported languages), this solution will make the app fall back to French, which is an appropriate localization. More appropriate than setting the user’s fifth choice, German, by hand.
What happens when an app launches is simple: The OS descends the users language list, as set in Preferences, until it finds a matching localization, and uses this language. The reason many apps default to English and not German is simply because English appears on most user language lists above German. There is no inherent language preference to the system. If you delete the English translation, only the languages you want to support are there, and of these languages the one higher on a user’s list is taken.