Complex sentence: As the area nowadays known as Finland was gradually incorporated in the Swedish Realm from the 13th century onwards, Swedish (and Latin) became dominant over Finnish as the most-used language of administration and higher education among the Finns.
Split into simpler sentences: As the area nowadays known as Finland under Swedish rule (Sweden-Finland), Swedish (and Latin) was the language of administration and education in the Swedish Realm. Swedish was therefore dominant over Finnish as the most-used language of administration and higher education among the Finns.