https://www.wahlinfo.de/pcwahl/index.html (German only)
There is also https://vote-it.de/?page_id=156 which is a bit more modern, but also shows this is a <10 person shop. Not at all what I expected.
Looks like their page on "security" completely misunderstands what computer security means. They write long-winded paragraphs only about data integrity and backup issues, and completely ignore any issues around actual attackers trying to compromise their system.
It's a niche market, I'm not surprised. I'd actually prefer if the software was paid for by the government and developed by a single (or a pair of) competent developers and open-sourced. It's probably possible to pull it of with that man-count, you certainly won't need more than a handful.
A small company isn't going to be able to compete there, Capita or someone will get it. They know how to go over budget by £Billions and still not deliver a working product ... I'm guessing it's similar across Europe because of procurement regulations for governments??