When Miss Fisher speaks to Karol in his native tongue, her words are not in Polish, sounding more like some gibberish.
In the opening scene, Miss Fisher and her offsider review several guns laid out on the table. The dialogue is good, comparing the Luger specs with the Mauser's, but the sub machine guns laid out amongst the other weapons are Russian PPSHs. They were introduced in 1941 for the Second World War, and this series is set during the late 1920s.