a standard Belarusian
Some of the older people in the films use local dialect. Watch the others and you'll hear standard Polish.
boletus. ...standard Belarusian. But that is what it is over there. Did not you notice an orthodox church complex on the background - with typical onion shaped turrets?
In towns it's about 50-50 Orthodox-RC, but there are lots of 'Catholic' and mixed villages. The people who speak in local dialects use Polish when they need to, and everybody knows Polish.