Stan , what deepakrr had said about the theming was right. But always ensure you use your own firmware for theming.
To edit the text languages, you can use the S1res Tool. The guide is
Here.
But for text editting, it is step 12 onwards.
You will need to locate the line that is German and change it to something you prefer. However, you can only change it to the same or lesser number of characters. Not longer than the original.