Not everyone understands stewie some people like Brian and unimportant characters can understand everything but most of the main cast only understands the gist of what he is saying i know this because the creator said so in an interview
Well, in my opinion, everyone except peter, lois, chris, and meg can hear stewie. There was the Stewie kills lois and lois kills stewie episodes where they could hear stewie, but that all turned out to be a simulation and wasn't real at all.(funny as hell)
Peter, chris and brian can understand stewie but for some reason the others cant i know this cause on season 6 there is a live show on the extras and seth macfarlane(creator and plays characterps such as peter, stewie and brian) says this but obviously in much more detail.
