As for actors with Slavic accents, there was number in Hollywood, many of them Jewish. There was Oscar Homolka. an Austrian born Jew . I don't know whether he actually spoke a Slavic language. I recall him playing a Soviet General in "Billion Dollar Brain. " In that movie there was also a scene where people supposed to be speaking Latvian were actually speaking Polish! It may be that Hollywood simply couldn't find Latvian speakers or there may have been other considerations. The average American would not have known the difference. Of course, a Slavic language speaker would have recognised the Polish. Latvian and Lithuanian are somewhat related to the Slavic languages. I would say that if the Slavic languages are siblings, Latvian and Lithuanian would be first cousins.
Which brings me to another actor, Charles Bronson. He grew up near Johnstown , PA. He spoke Lithuanian and Russian before he learned English. I couldn't see a tough guy like him playing a Pope!