Yes. Darktable is free/open-source software released under the GNU GPL, so downloading it from the official website or official mirrors is legal in Germany and other EU countries.

More generally, downloading software is legal when the publisher provides it and you follow that software’s license terms. That applies to both open-source software like Darktable and proprietary software like Lightroom when obtained from Adobe through legitimate channels. The legal risk usually comes from downloading copyrighted software from unauthorized sources such as torrents or unofficial mirrors.

So for Darktable, stick to the official site or trusted official mirrors and you should be fine. If you want absolute certainty for your jurisdiction, contact the developers or review the license terms directly.