I've tested your program and when you get used to the controls it works quite nicely. But before I was finished,
"Du hast gewonnen!"
appeared. All mini-cubes have been at their correct position but the orientation of the 4 corner cubes belonging to the green layer were still wrong, this needs to be checked, too!

I would not try to implement a hint system because of the amount of time that has to be invested into that. Instead, I would add a how-to-solve-the-cube tutorial so people learn to solve the goold old Rubik's Cube instead of repeatedly clicking the "Hint" button.

Keep on the good work!