Hi there!

Usually you state your income in the application letter, plus the adress of your employer, so that part is quite normal. Some appartments require a basic understanding of German so that you understand the house rules and can communicate with the caretaker and the neighbours who often don't speak any English, but it doesn't really make an expats life easier I guess.
It's not really usual that they ask if your wife is pregnant, but they'd still try to find out, because most places have a certain target group of people (ie. families, young professionals, older couples etc.) and they don't want a noisy young family. move into a house that is full of quiet tenants.
As for the legal question, I guess these questions are indeed legal, but would be considered discriminatory elsewhere.