

I speak French and German on a daily basis and German speaking people tend to use the du (or sometimes Du) more often than French speaking people. For both languages though, people tend to go more often with the informal forms in the rural areas and more with the formal form in the cities. This is for Switzerland and people in Germany or France may have slightly different habits.
You mean Interrail? It’s not a monthly pass, but you will probably find a pass that suits your trip.