Some more I18n updates

Summary:
* Pre-flight checklist translated to German
* Various typos fixed
* Hardcoded strings moved to support i18n

Thanks for reviewing and merging :laughing: 
@zumbrunnen You can merge `getString(int)` and `String.format(String, Object...)` by using [`getString(int, Object...)`](https://developer.android.com/reference/android/content/res/Resources.html#getString(int, java.lang.Object...)).