Thanks.
To get (or to write myself) a collator for natural sort not accessible via the API wasn’t my intention.
I definitely wanted to get the advantages of the very efficient sorting implemented in LibO, of its readiness to include the collation of many fields in the needed order (seemingly in one go), and of the related application of locales.
As so often: I don’t urgently need the answer. I mainly want to know.
In addition I remind of my opinion that every feature available via the UI (and sometimes based on UI usage even via recorded macros) should be implemented in a way making it also accessible by calls to API methods. In fact LibO should work as if it actually does everything by API calls itself.