Hi
Not sure what you mean – is a German calendar the same as a UK one but with German names for months and days or does it differ in some other way as well?
If it’s just different words, then I expect that translating the plugin into German would achieve the desired result. WordPress has a system to do translations (there are actually several ways) – You can see there has been a little translation done here https://translate.www.ads-software.com/locale/de/default/wp-plugins/wp-hr-manager/
There is an option on the right hand side (mid screen) to apply to become an editor or you can simply suggest translations from that site. All translation suggestions have to be approved by a WordPress authorized editor.