Hello, I really like the combination of DRM-free games and the Galaxy client, but why in the world do I have to input my Windows admin password like 20 times to install Witcher 3 together with all DLC's? Today i just change the audio language and I was asked 6 times for my password. What the heck...
Please, can you improve the installation of games with GOG Galaxy?

You have GOG installed in a a system folder e.g. C:\Program Files(x86)\GOG Galaxy? Move it to somewhere like C:\GOG Galaxy and the issue should stop. It's a Windows security setting, not a GOG setting.

Yeah, but why? I set the download and game folders to an other location, and besides I have plenty other programs in folders like "C:\Program Files(x86)\" (Steam btw.) which don't make any problems of this kind.

Don't just change where games are downloaded, move the entire GOG Galaxy folder out of Program Files. That should resolve the issue entirely. As for why other programs don't trigger this same reaction, I can't tell you. Windows has chosen to limit what GOG is doing to users with administrator privileges, and will require you to enter the admin password each time for as long as you keep it GOG Galaxy where it is.