The modern alphabet is the most widespread writing system in the Northern Kingdoms.
CD PROJEKT RED's The Witcher franchise[]
The developers CD PROJEKT RED use the Slavic Glagolitic alphabet as the basic human alphabet though Andrzej Sapkowski's novels do not suggest that the Northerners' alphabet resembles it. It is also worth to note that Glagolitsa is also used as alphabet for the entirelly different cultures, like Nilfgaardians or elves.
Below in the gallery, some posters with Nordic/Glagolitic alphabet.