Some years ago, I was sitting in a missionary meeting listening to an Aboriginal Australian speak of Christian work among his own people. He illustrated his talk with the usual slides of the outback, but there was one slide that seemed to me, totally out of place. It was a picture of his home, and there out the front was a genuine spear-carrying concrete Aboriginal garden gnome. If it had been a Homer Simpson or the like, there would some sanity in it.
Actually, some friends of ours removed their parents Aboriginal garden ornament and gave him an enforced holiday. For the next year they sent postcards home from "Neville" as he toured the outback on walkabout, and yes, even a picture of him with some of his friends. He even got married and was photographed with his new bride.
It gives me the cold shivers how we have so easily confused culture with the Christian faith. In the name of Jesus we have dressed half the world's native population in bras and bloomers. Every culture thinks it's superior and we Westerners are no exception. In truth though, the Christian faith stands quite apart from culture. In fact, if you want to give it a home, it's more Eastern than Western, but in the end, it's a faith for all mankind. God is not limited to cultural barriers.
There can be nothing but good in a tribal people coming to learn of the Creator's greater revelation of himself in Jesus Christ. This may affect some customs, for example, human sacrifice, infanticide.... Yet, in the long run, very little of a people's culture need be affected by a greater awareness of God the Creator.
There's one argument in favour of "westernizing" tribal people. How much weight it has, I'll let you decide. It has nothing to do with religion, although, out of compassion, many missionary groups do the "westernizing". It's the view that the spread of civilization is destroying the natural habitat of native tribes and therefore, if they're to survive, they must be educated and prepared for the onslaught. Much of this type of work is going on at this very moment in the Amazon jungle. Christian missions are literally saving tribes from extermination. The social results, though, are enormous. The people often lose their and identify and self worth. Yet, at the same, some do make the transition, but of more importance, some come to know their Creator.