I was in Germany for local elections four years ago, and it seemed like their lists of candidates were selected by something like a caucus of party members from the local jurisdiction. There was no primary.
Can't speak for other states of Germany, but in Hesse that's true only of very small parties and the Greens. Other parties will have a convention, with every ward group sending two delegates or three. (The situation for state and federal lists is similar, although there'll be more party members per delegate, and the Greens use delegates as well.) But yeah, at Green Party local election list selection, even for a city as big as Frankfurt, any party member can just walk into the meeting and vote.
It probably was a convention then. To an outsider, the functioning of a caucus and a convention are very similar. Only a careful observer would note that the convention voters are selected delegates. As a non-native, I hope you'll forgive my probable error. :S