>A Pokémon's Nature is determined when it is generated by the game: when it is obtained as an Egg from the Pokémon Day Care (by the player in Generation III—except in Pokémon Emerald—and Generation V, or by the Day-Care Man in Emerald and Generation IV), when it is encountered in the wild, or when it is given to the player by a non-player character.
No, different areas have nothing to do with the natures that Pokemon are given. The only thing that can change that is if the Pokemon has a set nature or if you have a Pokemon with synchronize.
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