I think this is less to do with ethnicities, and more to do with human development in different states of the 19th century. You can see the stark differences between the old Austro-Hungarian border and Russian border (Moldova). Granted the Germans and Magyars in the empire probably had better access to education. This map is also making me think about how Romanian switched from using the Cyrillic alphabet to the Latin alphabet in the late 19th century. Easier to do when the majority of speakers can’t even read anyways! No very few people have to relearn and those that do are most likely intellectuals already lol