You're allowed to think whatever you want, but you're not allowed to publicly say it, unless it's within the bounds of legal opinion (which are pretty wide, it's not like you're only allowed to state one opinion, but they are also clearly limited). Insults, while clearly speech, are forbidden and it's part of the penal code (up to one year in prison, but it's usually settled by fines), so not a civil matter.
What Germany certainly doesn't have is a culture of free speech. It's important to remember that the last totalitarian dictatorship in (East) Germany only ended 30 years ago, and there was very little freedom of speech there, and the limits were violently enforced.
German culture highly values conformity, and it's no accident that the spiral of silence was proposed by a German researcher (Elisabeth Noelle-Neumann), it essentially argues that people check their views for majority-compatibility and will stay silent if they find that they're not accepted. Because they stay silent, others who think like them will find their ideas not accepted in the majority and will also stay silent etc etc, as an act of social survival (which it certainly is in Germany, but isn't as much in countries with a culture of free speech).
> speech absolutism and value neutrality of the constitution of the Weimar Republic
The Weimar Republic had largely the same fundamental laws regarding freedom of speech as the Federal Republic of Germany today: you're allowed to say what isn't outlawed by a law, and you must not be punished for saying it.