Interpretation of "the affairs of [these] subjects had to be administered too"

Question

I am translating a book, and there are some confusing sentences:

This was becoming essential for the cohesion of the realm, not least since its linguistic and cultural plurality had been extended substantially through the conquest of new provinces. In the sixteenth century its languages were Swedish, Finnish, Estonian and Lappish (Sami). In the following century the affairs of German, Danish, Latvian, Polish and Russian subjects had to be administered too.

A History of Finland, HENRIK MEINANDER, page 41

Actually I cannot catch the bold sentence. My interpretation is that maybe some linguistic elements were added from these languages, but I am not sure.

Answer 1

This is subject in the sense of 'citizen'. There were people of all those nationalities living in Finland. (Presumably the thing that was becoming essential was administrators knowing numerous languages?)