The letter ‘K’ as a mark of origin?

[RobGea]

in which linguistic area were these writers active if K is the dominant letter?

You could check wikipedia letter frequency.[1]

Code:

Language  Letter_K frequency (%). Finnish       4.973% Hungarian     4.85% Turkish       4.683% Polish        3.411% Danish        3.395% Icelandic     3.314% Swedish      3.14% Czech        2.894% Dutch        2.25% German       1.417% English      0.772% French       0.074% Welsh        0.0396% Spanish      0.026% Portuguese   0.015% Italian      0.009%

Data from wikipedia
[1]https://en.wikipedia.org/wiki/Letter_frequency