Complex sentence: The Poles are the nation of the Vistula, and their settlements extended from the sources of the river to its estuary at the Baltic Sea; there is no other European nation centred to that extent on one single river.
Split into simpler sentences: The Poles are the nation of the Vistula river, and their settlements extended from its sources to its estuary at the Baltic Sea. There is no other European nation centred, to that extent, on one single river.