I for one think there should be some modeled xenophobia in the game. That's just how it was at the time. If a chinese adventurer conquered lands in western europe somehow, they would be targeted and isolated. If he was rich it would help, but he would still have to work much harder and for the rest of his life to convince people he had become one of their own, and the adopting culture mechanics does not cut it, as his ethnicity would still be very alien to both his subjects and his peers. Being fluent in the language should be just one of the very necessary steps.
I can't speak for how medieval chinese people would react to an european adventurer conquering land in China, as I don't know enough about their world view. But he certainly would also be seen as a barbarian, and would have to work for the rest of his life to convince the people of his commitment to becoming one of their own.