Thank you for the question. I should have written this in the documentation more clearly.
The application is multi- (okay: bi-)lingual thus you don't need to change anything in the application, nor the server.
It uses the language of the user, as it is set in the preferences of the Notes client:
File - Preferences... - Regional settings - Content Language
(Datei - Vorgaben.... - Ländereinstellungen - Sprache des Inhalts)
You can test it with your German client by adding English as Content Language and move it to the top. Restart your client, open the password safe and the user interface should be in English.
Thanks
Thomas