Is seating distribution planned in airliners or is it left to casuality?
People tend to travel by themselves or in small groups. They sit all over the airplane, either selecting their own seats or having them assigned by the airline's ticketing system. When you go to select your seats, the system shows you what seats are available, but it does not show you other passenger's ethnicities. Airlines also don't necessarily know passengers' ethnicities when assigning seats in advance, except to the extent that they could conceivably guess based on names (they may know what country's passport someone is traveling on, which is not the same as their ethnicity, but even so, they may not know this information until well after a seat assignment is in place).

So lots of people check-in and have their seats assigned basically randomly. The expected result is that small groups of people of different ethnicities would be mixed throughout the plane fairly randomly. I'm not understanding why you think this is something an airline would deliberately need to do, as it would already happen naturally. 

Using ethnicity for seating purposes could also violate anti-discrimination laws in many countries, if this was something an airline even wanted to do.

It is sometimes the case that a large group of passengers largely of the same ethnicity will book a block of seats together as part of a group booking, such as a Japanese school trip or German musicians. Such groups may, if space permits, be seated together, and if the group is relatively ethnically homogeneous, you may find a number of travelers of one ethnicity seated together for this reason.