Complex sentence: While what today constitutes the Republic of Finland was then a set of provinces comprising the eastern part of Sweden proper, of which the southern part, more suitable for farming, was known to the Swedes as "Österlanden" ("the Eastern lands").
Split into simpler sentences: While what today constitutes the Republic of Finland was then a set of provinces comprising the eastern part of Sweden proper. The southern of these, more suitable for farming, were known to the Swedes as "Österlanden" ("the Eastern lands").