@gebeer, yeah I said that but also said that another selector would be required and that I didn't know if that would work. But I just tested and it works fine.
What you need is this type of selector:
"select_page=page.select_country, select_page!="
Setup
"select_page" = would be page single page select field on your option pages to select its "reference", like "Zürich" has selected "Schweiz", and so on
"select_country" = would be a normal page select on current editing page to select from a parent of countries. Nothing special here.
Then use a page select to select cities that are dependent on what country is selected:
"select_area" = would be a normal page select on current editing page to select from cities. Here you would use the custom selector option for the input like: "select_page=page.select_country, select_page!="
And so on. You can go further and create a page select that is dependent on the selected city and add a "select_event" that then uses same selector but with using "page.select_city":
"select_page=page.select_city, select_page!="
This allows dependent relations using a page select instead of a parent relation. This is only for backend.