Complex sentence: Central Croatia is the most significant economic area of the country, contributing 54.5 percent of Croatia's gross domestic product, the city of Zagreb being the largest population and economic centre of the region and the whole country.
Split into simpler sentences: Central Croatia is the most significant economic area of the country, contributing 54.5 percent of Croatia's gross domestic product. The city of Zagreb has the largest population and is the economic centre of the region and the whole country.