There are some cultures which have a hard time surviving which makes perfect sense, but why is Frisian guaranteed to die out even when ruled by a independent Frisian King/Emperor? It should be treated like Pictish and Visigothic and so on and be able to survive under some circumstances - to be fair Dutch should only really spawn from Frankish culture in de jure Frisia, not Frisian culture as well, but at least this change would make it theoretically possible for Frisian to survive.
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