Modifications in strings.xml files should only be done in English, in the file res/values/strings.xml. Any update for other languages, including German, must be made in the ownCloud project at Transife, at https://www.transifex.com/projects/p/owncloud/ . Create your own account there and ask to join the German translations team(s).

Modifying directly the files here breaks the workflow, creating conflicts any time that the translations are updated from Transifex. These conflicts are specially bothering, since we can't fix them if we don't know every sinlge language.