No you cannot go to other regions in Pokemon Pearl or Diamond version but in Pokemon Heart Gold and Soul Silver you can go to the Jotoh region.
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Unfortunantly you cant go to other regions in Pokemon diamond and pearl. its not like leafgreen,firered,gold,silver and crystal version where you travel to different regions. You can't, Sinnoh is the only region available in the game.
It is only in HeartGold & SoulSilver and Gold & Silver you can go to different regions :/(and Crystal)
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you can only go to kanto and johto
No. You can only pick Turtwig, Chimchar, or Piplup. If you want starters from a different region, there might be an action replay code for it.