Well, in
my setting there is a human world (ours), and a non-human (Eldeven) world that has almost no humans in it.
The humans that do live in it exist in the equivalent of the British Isles though there are only about a few hundred of them at most (counting part human offspring). They are mostly descendents of the survivor's of Arthur's Court (Kamelod) though only a few of them are aware of this, most of the original survivors having died off. And no human player characters exist in the other world. Just as there are no non-human player characters in our world, except on those occasions when various characters visit each other's world(s).
These two worlds interact from time to time, in secret, but humans have no direct influence on the other world, although because of recent events human religious ideas are having an effect upon the other world.
So when playing the other world human culture has very limited impact, and the same is true vice versa, our world.
Though both worlds have an interest in the other.
This keeps either set of cultures (for with the Humans you have Romans and Vikings and Europeans and Barbarians and Syrians and Persians and Arabs and Christians and Jews and Muslims and Pagans - and with the Eldevens you have Sidelh, Lorahn, Jukarn, Fyel, Avafal, and the Adharma [Giants] and their various cultures, governments, and religions and so forth) from overwhelming the other, and yet they do interact in different ways.
I of course didn't have to invent any human cultures, religions, societies, or religions, merely research them for the time period involved.
For the Eldevens I had to create a different culture, government, society, religious view etc. for each race, ethnic group, or however it was that they arranged or differentiated themselves.`
As for what the players do. They play in a party
that is entirely human, or one
that is entirely non-human.
Though if things continue as they went last time then it may end up that a sort of hybrid team develops.