If the instructions are in German, but the kit is from China, I wonder if it's actually real German or something Googlefish made up.......
I have 2 different sets of Chinese brake light/turn signals combos, haven't tried them yet, but yes, they do look rather cheap and I'd be very surprised if they last longer than maybe a few months at best.
Question is, do you actually need such things in terms of the traffic you encounter ?
Never forget that that's a bicycle you're riding.....
Currently, I only need lights once a week and am using a cheapie set of clip-ons, the red rear one has 6 steady/blinkie settings, the front one 3, the front has a tendency to shut off going over larger bumps.
Not great, but ok for the price - something like $ 10 - and enough for city purposes.
If you ride in rural areas with no street lights or ambient illumination form buildings, you'll need something FAR more powerful (and heavy).