Konyok:
“f one were to draw a line from Rostov to Krakow a smooth linguistic continuum would be revealed. There are no sharp lines; Russian grades into Ukrainian which grades into Polish. Turn west at Kyiv and the endpoint is Slovak and Czech.”
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Quite true. As you travel criss-crossing that enormous area, the language is mutually intelligible as you move from town to town, even as you cross borders. If you grew up Slovak, which is linguistically kind of in the middle, you can travel through perhaps a fourth of Eurasia and communicate to a fair extent just by speaking your relatively obscure native language.