Complex sentence: Since the 1990s, large numbers of Albanian immigrants have arrived in Greece, forming the largest immigrant group (443,550 in the 2001 census) unofficial results claim 1,000,000 Greeks are of Albanian descent either by ancestry, marriage or immigration.
Split into simpler sentences: Since the 1990s, large numbers of Albanian immigrants have arrived in Greece, forming the largest immigrant group (443,550 in the 2001 census). It is unofficially claimed that 1,000,000 Greeks are of Albanian descent, either by ancestry, marriage or immigration.