Hi there, thanks for any support.
You would need to fork or extract the source from the github project https://github.com/reportico-web/reportico
and go into the folder language/packs, copy the folder en_gb to say de_de and edit the strings in there. Then to try it, copy that de_de folder up to the language folder and change to German in the admin page.
I would say there are some strings in the code which are hardcoded to english which should be converted over.
If you want to send me them or send a pull request in and be a contributor i can help you merge it.