The mod doesn't seem to work with the latest version of the game (1.14.3 at the time of posting). I started a new game as William II but couldn't find the option to take a concubine. Is there a possibility of an update? I would love to try this out!

It would be possible and the word concubine appearing is not ideal, but I set it up this way to represent the way that Christian society viewed the institution. The Church commanded a status as the only arbiter of legitimate marriage and these unions fell outside that. When Richard I wanted to nominate his son Robert to the Archbishopric of Rouen, he had to remarry his more danico wife Gunnor according to Church custom to legitimise his son.

Is it not possible to have 2 wives instead of having a concubine?

Just updated for the new version!

ok

Norman = A Norse French Hybrid
Norse = A modern general term for all vikings, but actually referred to the people north of the Danes, and west of the Swedes and Geates.. modern day Norwegians.
Dane = Danish
Swede = Swedish, but mainly north east Sweden
Geate = South east Swedish tribe/kingdom/people

Norsemen/Northmen = Danish names of Norwegians.

Nordmann = Modern norwegian word for Norwegians.

Norman = Nordmann = Norseman = Norse = Norwegian.

Norman is therefore a hybrid of Norwegian and French. Some Danes probably too, but lets not forget that Denmark owned Norway 400 years and have hijacked much of Norwegian history and still try to downplay Norway and its actual importance as a rival to Denmark.