Is your feature request related to a problem? Please describe.
I have a lot of German language content in my Library. Calibre offers a metadata surce plugin for the "Deutsche Nationalbibliothek (DNB)" (German National Library), which I really like and is very useful for German content and well curated. Link to the plugin: https://github.com/citronalco/calibre-dnb

Describe the solution you'd like
I would like to have the option to select the DNB plugin as metadata source.

Describe alternatives you've considered
The presently implemented metadata sources work well for English content but not for German language content.