How does Ck3 change court language?

Find cultural options. There is a plus sign in the language of the cultural interface. The plus sign can change the language.

Language is a very interesting game, and court tutors play a very important role, because they can help you and courtiers learn new languages, thus triggering new events and interactions. The act of learning a language is a conspiracy. Competitors may try to sabotage your efforts, but learning a language may also lead to love misunderstanding in your court. If you are born in a culture, you will instinctively know its language. You can learn the language by observing the roles of other cultures and decide that I want to learn the language they speak. This may be your bannerman or your superior Lord. Oltner explained that the reason for this is that when the activity is based on roles, we can incorporate very interesting flavors into it.

Language is "a small part of extensive revision of cultural system", as a sub-function of culture itself. Being able to speak a foreign language can halve the goodwill of rulers when interacting with characters and provinces, which will be very helpful when they inherit the land of different cultures.