But I am not stuck to the Windows/Borland world, instead for the Pi simply the beginner-friendly and handsome C(++) GUI programming tools are missing (and GUI programs should apply to common C99/C++11 datatypes and functions, no sort of qstring gibberish)
If what you want to use is not available on the Pi and your refuse to look at anything else then I guess you are stuck in the Borland/Windows world of the 1990's.
Only a handful of years ago I was in the same position. My idea of programming was C/C++ and assembler. I had spent decades programming in "real" languages that compiled down to real machine code, Ada, PL/M, Coral, Pascal, etc. I had no idea to stoop to any silly, slow scripting language that people used to make annoying animations in their browsers. If I needed a GUI it was done in qtcreator or such like.
Then one day the boss decided he wanted an animated data visualization that I had created in Qt to display in the browser. This thing took in a stream of data at 20 updates per second and updated a few hundred objects on screen. As none of us knew anything about web technology, JS, HTML, CSS and as what we needed to was hardly possible at the time, and as we had no desire to do it, we hired a guy to do it for us in FLASH. There was a six week deadline to have it ready for display at an expo.
I now had little to do for the six weeks before the expo. I had vaguely heard of some new web technologies. Like webgl for hardware accelerated 3D graphics in the browser, websockets for real-time communication from server to browser, the V8 JS engine to speed up JS in the browser. So, turning on the "experimental" flags in Chrome I set about experimenting with these things, just for fun...
It also turned out that the guy building the FLASH version did not get it working before the expo. but my "experiment" was working and ended being demonstrate there.
There has been no native GUI creation in my life since. I had found something much easier, more flexible, and far easier to deliver.
What's the point of my overly long rambling here? Well, in short, if you are proficient in C++ you can certainly pick up JS pretty quickly. You may be pleasantly surprised.
At least you will find plenty of tutorial material in German I'm sure.