First of all - most people also in the US obviously don't know that there are exceptions from the First Amendment, excluding certain categories of speech from protection. Second, the First Amendment at the time it was created had a much more limiting interpretation of this freedom - e.g. insults were actually punishable.

Additionally - freedom of speech has never been absolute - it was limited in Ancient Greece, where it originated, it was limited as it was declared in the French Revolution (which was used as the template for the First Amendment), and in the International Covenant on Civil and Political Rights - which provides the internationally valid definition of freedom of speech, its scope is also limited.

As opposed to the US, in Germany, the legal system is built consistently from the most generally valid principles to the most specific ones. The most universally valid principle is the one of the human dignity - this is why it's prominently placed in the first sentence of the German Basic/Fundamental Law. Everything else is more or less directly derived from the First Law, making them subordinate to it - thus if freedom of speech violates someone's dignity, the dignity is more important and has to take precedence, limiting the freedom of speech.

And this is consistent and concordant with the international definition.