Vojvodina, the northernmost province of Serbia, is home to, according to Wikipedia, 26 ethnic groups and 6 languages. While I’m aware there are plenty of areas with great cultural and linguistic diversity (say, the Caucasus or New Guinea), I was under the impression that a large reason for that diversity is the presence of mountain ranges. But Vojvodina is largely flat, with its highest peak reaching only 641 m. It’s mostly in the Pannonian Plain. How did such an area manage to achieve such tremendous diversity?
Since the medieval times, present day Vojvodina was inhabited by Hungarians and to lesser extent Serbs. After the Ottoman conquest, that started in 1526 (and went well in 1550s), the Ottoman govenrment encouraged the Serbs and the Romanians to migrate here (as the region was depopulated): this is the period when the Serbs became actual majority.
The territory of the present day Vojvodina came under Habsburg rule in 1690 (roughly western half) and in 1718 (roughly eastern half). Charles VI, as Hungarian king, encouraged poor German farmers to migrate. This did not give much result, nor did Queen Maria Theresa's more extreme attempts (to move in bandits and prostitutes from German lands). Now King Joseph II changed the strategy, giving a set of privileges to those who move into present day Vojvodina. This created the atmosphere that allowed for immigration: for example, the protestants were given freedom of confession. Following the period from 1780s, many homogenous settlements of Serbs, Hungarians and Romanians became mixed, while of course a large number of entirely new settlements were built.
This is the most concise answer I could've provided. If you have additional questions, please let me know.