Henry Ford created the 40 hour work week because it made sense financially. He was able to coax workers away from his competitors. They had to follow suit.
We will get a 32 hour work week when it becomes economically possible. Someone, like Henry Ford, will create the 32 hour work week because it makes economic sense to do so at the time. Competitors will have to follow.
The government enforcing a 32 hour work week at the same pay will simply cause inflation. Works will receive, in real wages, only 80% of what they currently do (no matter how many times the government increases wages).