From Italy I downloaded DTTG from AppleStore. The version downloaded is in German, not in English. I tried to reinstall and to download again. Many updates are intervened since my first installation, but it was always impossible to change the language. For me it is not a big problem, because I know German quite well, but …
Have you checked your language order in the system settings? If German is higher in the list than English any app that offers a German localization will use that as a preference.
In general apps have no control over the localization except for that they offer it. iOS will choose according to your preferences.