In my twenties I lived in multiethnic suburbs of Washington where the norm was for twenty-somethings to rent the bedrooms and share the kitchen in a house/flat, and yes, these houses were typically multilingual. An official house language might be set by the oldest occupant and used when that person was at home, but housemates might use whatever other languages they had in common. My husband and I had learned English first but sometimes, either after speaking or reading other languages, we spoke Spanish or French at home. (I'm very wary of saying I speak French, but I do read it and will speak it with people who are either kind and sympathetic, or desperate.)