Titles are shortened to whatever is in the Short Title field. Zotero Defaults to splitting this on a colon (and that's the standard title/subtitle delimiter in English, so might be worthwhile to consider changing that in the entry), but in either case you can edit the Short Title field to whatever you want manually.

I realise that I can use the Short Title field. However, I'd love to not have to enter short titles for every item, especially, if it is theoretically so easy. I could still use the short title where things need to be changed.

I am working with German texts, periods are the standard there, I cannot change that.

How do you mean Zotero defaults to splitting, when does it do that?