Hi Andreas,
The hotel module languages are defined in this folder:
C:\ProgramData\HotelModule\Data\Language
There is a file named "HotelLanguages.xml" containing the definition of files, for example:
<languages>
<English culture="en">strings_hotelApp_en.ini</English>
<Deutsch culture="de">strings_hotelApp_de.ini</Deutsch>
</languages>
Then you have the file "strings_hotelApp_de.ini" containing the texts in German.
Can you check if your system has these files and German is included?
If you need to make any change to it, you will have to restart the web server (IIS / Abyss) in order to reload the information.
Kind regards.