Lingua Franca

Every royal court in the WORLD speaking your court language - one of the game's true monsters, and NB the wiki flatly states no ruler-designer characters. Charles the Bald (867) is the platform: his claims on three weak relatives' kingdoms set up Restore Carolingian Borders, a huge de jure empire with early Primogeniture. From there the realistic method is subtraction: destroy every other royal court by conquest (the By The Sword tradition removes the once-per-lifetime cap on kingdom holy wars). AI courts of a different religious group or with distant capitals will essentially never adopt your language voluntarily; for vassal kings, convert their capital county to your culture so they follow suit, and if a stubborn one still won't switch, revoke the kingdom and hand it to someone of your culture. There's also a report of a completed world not firing it - between that and the designer exclusion, use a historical character and keep expectations patient.