When I was in Poland in 2001 a friend of mine who had been studying Russian, figured he could speak to the Poles in Russian since until 1990 or so it was mandatory to learn Russian in Polish schools. As soon as he uttered one phrase in Russian people would stop acknowledging his presence. Another friend was trying the same thing with German and no one cared and would chat with him in German, of course much less people knew German.

Historically the Russians along with the Germans have been the Poles major enemy. In 1939 I doubt they would have been happy to seen Soviet forces even if it was ostensibly to save them from German forces.