I honestly don't see how you could model something like this without using ethnicity in some way. Personally, I'd be inclined to base it more along the lines of a negative interaction modifier if local groups aren't of the same ethnicity. It's not so much giving one ethnic group a bonus as it is modeling the difficulty often present when it comes to getting two different groups to work together or trust each other. The modifier could be negated over time (possibly by steps taken by the outside nation), or it could actually be enhanced (due to a number of factors).