Level: HardYour culture doesn't count. Start in 867 as High Chieftain Radomil of Lusatia. Slavic cultures start at 50% acceptance towards each other in 867 and Polabian culture offers the best start while also lacking the Staunch Traditionalists tradition. If you get a royal court use the Administrative court type.
You can also form a hybrid culture. By doing that you will get 100% acceptance with both cultures and they will both count for the achievement, because of your new culture.
Credit to
Bayek von Kuwaq on Steam for his
guide.