In the 700s, a majority of the Byzantine Empire was either overrun or inhabited by Slavic speakers, including Greece, the heartland. Before this, the language of the emperor had changed from Latin to Greek (maybe later, if I am likely wrong). Your challenge is to change the language of the emperor, and the bureaucracy to Slavic.
Your second challenge, not necessarily in the same TL as the first, is to get a Turkish Byzantine emperor and government, not merely the Islamic heirs of the ERE, Rum and the Ottomans. Doesn't matter their religion, but you get ebola a cookie if they're Catholic or Muslim.
Also, again not necessarily in the same TL in the others, what about a Norse emperor? The bureaucracy doesn't have to speak Norse, though.
Your second challenge, not necessarily in the same TL as the first, is to get a Turkish Byzantine emperor and government, not merely the Islamic heirs of the ERE, Rum and the Ottomans. Doesn't matter their religion, but you get ebola a cookie if they're Catholic or Muslim.
Also, again not necessarily in the same TL in the others, what about a Norse emperor? The bureaucracy doesn't have to speak Norse, though.