Complex sentence: Despite Polish productivity rising to 2/3 of those in the West, the wages in Poland remained low, at 1/3 of that in the West, this has contributed to the flight of Polish population from the country.
Split into simpler sentences: Despite Polish productivity rising to 2/3 of those in the West, the wages in Poland remained low, at 1/3 of that in the West. This has contributed to the flight of Polish population from the country.