As an example, for France. I am using:
Chargement de la base des codes postaux. Source : INSEE/La Poste sur unpkg.com
This is the most important one, a merge of municipalities with their unique code with the postcode official database. While EBT uses the postcode to fight homonymes, the postcode can't be a reference because it is often used by many municipalities. So I need a unique (INSEE) number as a reference. There are updates every year (except on municipal election years) so I am lucky this one has been compiled and is up to date but I had to convert the format and can't automatically grab the latest.
Chargement des données des altitudes moyennes. Source : Opendatasoft (Licence Ouverte) (2013).
The only altitude database I found is 7 years old, this means there are possible 2000 municipalities with an approximate (wrong but not too much) height.
Chargement des données de superficie. Source : sql.sh (CC-by-sa) (2016).
Surface database is also older and source of aproximations
Chargement des données de population. Source : INSEE (données de 2017 publiées en 2019).
Luckily the population file is a public database that is updated every year
Carte des communes de France par région
The second most important is the geojson file with the limits of all french municipalities. This is a huge work and it is splited by regions and I load directly from the project:
https://github.com/gregoiredavid/france-geojson/ My main problem is that this data is one year old so there are about 230 municipalities that are not correct. I would love to be able to update it, it is only about merging polygons but I don't have software for that. What is important is that the geogson data is using the same reference as my first database, the unique INSEE code. With this it is easy to mark the map with valid municipalities.

For Luxembourg, I went on the
Geoportail.lu and I had access to all needed data (often available after I requested it by email). I got a bit of help to build the map from the API managed by the public service of the Grand Duchy. I hope that every country will have their data available like this very soon.

For Portugal I think this is good to have a list of concelhos with their postcode, a possible unique identification number and possibly another list with the freguesias and their concelhos. Then, there need to be a file with this same list and their borders in gpx, kml or geojson. When I look at wikipedia and OpenStreetMap, I don't feel that such file exists.

For the Netherlands (as Henk asked) The dutch Wikipedia team is quite good and update the
kaart van Nederland met gemeentegrenzen every year at minimum. I suspect there is a gpx map available somewhere but haven't looked yet. The list of municipalities is easy to get aswell but what would be needed for Eurobilltracker data is the list of dorpen with attached municipalities. I know I have notes recorded in Abcoude, in Mijdrecht but do I know I have notes in de Ronde Venen?
Again I would first like to review and improve this software and I'll then be happy to share the code with some of you who want to make the same for other countries.