The functionality of Darktable is very good so I thought I would switch from Lightroom to Darktable. Can I download it in Germany or any other European countries which have strict legal standards for downloading software?
Darktable is free software and it is perfectly legal to download it in entire EU. About download in Germany - download the software from official web site or from official mirror site.
About Lightroom - it is legal again if you download it from Adobe web site. And its legal to use it on the time of trial period.
Do not use p2p/torrent to download LR software!
"darktable is released under the terms of the GNU General Public License Version 3 or later." -https://www.darktable.org/about/
It is free software, unlike Lightroom. As a point of clarification, it is not illegal to download Lightroom either, as long as you have paid for it. If I'm reading between the lines of your question correctly, you are asking if downloading darktable is breaking intellectual property laws. (As downloading Lightroom without paying for it would be) The short answer: No.
Usually, if the official web site of any software has download links, it is legal to download. This includes proprietary software (eg, updates and trials) as long as you abide by the terms of the license.
There have been exceptions in the past. So if you have any concerns, you should contact the developers directly.