A post I created recently had some of this information answered in a little more detail (credit to geb and adabru for some of the information below) which may be helpful to read, bookmark and check back for updates: Eye Gaze Tracking With Head Tracking Solutions On Linux
One of the more productive and easier options to set up according to adabru, https://handsfreecoding.org/ and many others I've come across online: https://talonvoice.com
Appears to work offline for analysing spoken words (see 7. Privacy): https://talonvoice.com/EULA.txt
You can use the Vosk engine in Talon for German support if you pay $25/month, at the time of writing this, for the Beta version (see Vosk and the Talon community wiki for languages supported):
There is also a free version of Talon but keep in mind that Talon isn't all open source code.
I would give Numen a hard look. It's free and open source software that uses Vosk which supports German. Looks like a very good option if you primarily use keyboard-centric programs (some are listed in the link): https://git.sr.ht/%7Egeb/numen
There may be other Vosk projects that suit your needs at: https://alphacephei.com/vosk/integrations
You can use Dragon with Talon but Dragon is native to Windows. So as far as I know, you would likely need a Linux virtual machine in Windows or have to use Cygwin in Windows (see https://handsfreecoding.org/using-dragon-with-linux). Probably not what you're looking for, but Dragon supports German and I think I remember Nuance told me Dragon works offline for analysing spoken words (I would double check this). You could also use Dragon with Dragonfly, which is mentioned at https://handsfreecoding.org/. Dragon is going to cost you about $300-$500 (see https://talon.wiki/speech_engines/) and it's proprietary. I personally wouldn't recommend Dragon from my experience with it and it wouldn't be my first consideration.