Hi Gilles.
No, a custom country field is currently not available.
In the current 19.1 we use something like this (e.g. in adress book):
We will discuss this internally to see if this is feasible with reasonable effort.
If we include this, but certainly not in an old version.
You can certainly program this into it yourself. If you know how it works and you have a suitable installation.
Can you please describe what you would like to have the country field in the calendar for and where it should be displayed again?
Maybe there is an alternative solution…
And, as @AlexS already asked: Why do you use such an old version?
It’s also a security risk. In EGroupware (few) security problems have been fixed since then and your PHP is not maintained anymore:
When you need fast and competent support for an update:
Stefan