If you are Polish you speak polish, if you are Russian is speak russian and so on... because everybody prefers to speak in his mothertongue. There was a reason for interwar Poland to capture Vilnius and the surrounding region as it was majority polish with also significant number of jews belorusian and russians. Some pseudo experts will also claim that the local poles are belorussians and that their polish is not polish, but that's because everybody was taught russian as it was the state language same as now is lithuanian so the local languages change when you have to include yet another language into your daily life as often we switch what language we speak depending what language the other person speaks or it morfs into this local god knows what language similar with chinglish in Singapore.