Yes, I know that the conditions are somewhat implausible outside of CK2 - this is chiefly for a little worldbuilding exercise.
Basically, the scenario (lots of handwaving here) is this - Vikings serve as mercenaries in North Africa from some Muslim potentates in the area, don't get paid, they mutiny in the 860s CE, seize power and basically become a warrior aristocracy (with a few colonists in a region in the vicinity of Hippo Regius.
In the 900s, Christian missionaries from the Latin West convert the king, let's call him Gunnar, and most of his court, which slowly spreads to the common people, and that by around the 1050s at least a large plurality are Western Christians.
What is the aristocracy likely to speak?
What about the (previously Arabic and Berber, and to a lesser extent, African Romance-speaking) common people likely to speak?