CALMEFORET First, he did not call the people retarded: he spoke of their retarded world view.
Second, I see his response neither as childish nor as lacking in propriety.
Third, people who make the association of Mein Kampf and Mein Kopf most likely do not know German (or at least do not know it well - I for one did not make this association and it took me a moment even to understand what the discussion was about); in that case, any intelligent person would first wonder what it means in German and realize that mental punning in a language one doesn't know is likely to be totally off the mark, rather than jump to conclusions that show nothing more than his/her own ignorance of German. Though it might make sense commercially to take into account the less intelligent "sensitive" people, it is certainly defendable for the developer NOT to cater for such people, who in my opinion DO have a limited (if not retarded) vision of the world.
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