Update text_strings.xml

Hello serso,

as the app updated a few days ago, i recognized the translation. As german is my mother tongue, i would like to contribute a enhanced translation.
I have redone the german translation in /values-de/text_strings.xml.
Feel free to contact me with further questions and be so kind to notify me, when the translation is merged into the project. When i have time, i will also work on the other files.

Regards,
Alexander Schmidt from Bavaria