Earlier quoted context omitted.
A Yiddish scholar once said that "a language is a dialect with an army and a navy." It's really true. One might think that mutual intelligibility (ability for speakers of one to make themselves fully understood by speakers of the other) would be the standard for differentiating languages from dialects, but the distinction really ends up just being political. The spoken forms of Hindi and Urdu are entirely mutually in…
I'm not convinced your Polish-Russian-Ukrainian example is correct. My wife is from the west of Ukraine, and her home language is Ukrainian. However, she is totally fluent in Russian, as to be expected from someone growing up during the Soviet era. Both Ukrainian and Russian are written using the Cyrillic alphabet, whereas Polish uses Roman letters. She finds Polish fairly easy to understand, from an aural point of v…
Russian and Ukrainian are East Slavic languages; Polish is a West Slavic language. None of these three languages is mutually intelligible with any other, but Russian and Ukrainian are much closer to each other than other pairings. For example, Polish words generally have a fixed stress position, which makes Polish speech sound foreign and bizarre to unaccustomed Russian/Ukrainian ears. Many features of grammar and syntax are also much more similar between Russian and Ukrainian compared to Polish.
This basic picture is clouded a little by the split between east Ukrainian, which is lexically much closer to Russian, and west Ukrainian, which is farther from Russian and has very many lexical borrowings from Polish, because west Ukraine had been a part of Poland for a long time. So someone from the west of Ukraine will hear many familiar words in Polish speech, and will find it easier to understand Polish than someone from the east of Ukraine. That's sort of an icing on a cake, however; it doesn't change the basic fact that Russian is still much closer to their native language. If you could find someone from the west of Ukraine who had never been exposed to anything but their native language (and it's rare, as you mention, because almost everybody knows Russian at least to some extent), they would almost certainly find it much easier to learn Russian than Polish.
(my native language is Russian and I grew up in east Ukraine)
(and don't get me started on Belorussian)