Actually, a missionary is generally a type of *visitor* who sees no reason to assimilate (or even to show ordinary politeness) toward the local culture while they’re present. They are often not immigrants at all, in that they don’t plan to remain in the county permanently; even those who spend a whole career there will often retire back to the homeland they came from, or send their children back there. Their money comes from an organization back in their homeland as well, they often never do apply for a work permit in their temporary nation, and just keep functioning off tourist visas, which are often a LOT easier to get.
They don’t assimilate because they see themselves not as immigrants at all, but as foreigners who are visiting on what amounts in their eyes to an extended volunteer travel arrangement. The arrogance of this doesn’t *just* lie in the fact that they try to change the local culture; it’s also that they don’t ever recognize themselves as living there —even in a place they spend the bulk of their adult life —at all.