Multi language: The template could not be found

Just wanted to give the multi language feature a try, but failed instantly with the debugger:
»The template could not be found«
Line 95 – throw new Exception(‘The template could not be found’);

Of course i’ve read the docs and prepared the start page with .en.txt and .de.txt-versions. Unfortunately i’m using a template out of the box (KirbyBook), so the transparency is difficult.
Are modifications to the template itself neccessary, too?
Or is there an easier solution for my task? An english start page for my international guests would be just fine, i don’t need a translation of the whole site.

Thank you.

No, you have to define your languages in the config.php file. No need to change your templates. Are there any template files in your template folder?

Yes, changes in the config.php were done. There are seven templates in /site/templates.

Hm, I guess because of the KirbyBook template, there are no steps to reproduce?

Which version of KirbyBook is that? Are you using the KirbyBook template as is or have you made any changes to it?

I have a version of KirbyBook in my files with no default template. Maybe that causes this error?

What is your Kirby version?
What PHP version?
What environment?
Have you added any plugins other than in the original theme?