Sinnoh Dex: you can get palkia/Dialga/giritana north of mt. cournet depending on your Pokemon game is the legendary you get then you can get the "three mystic being's" azelf uxie and mespirit at Lake Acuit, Lake Valor, and Lake Verity. mespirite is by the lake at your home town at Lake Verity, Azelf at Lake Valor, and Uxie at Lake Acuity you can get dialga by trades.
National Dex: you can go get Giritina after you complete the game and get the national Dex. (note it will not be in origin form) well you don't need to beat the game just complete the pokedex. the next one is heatran you can find that after you do 2 things get up to stark mountain talk to the guy with red hair (not flint) i forgot his name but youll go through the mountain with him then youhave to go back to the resort area and go into the buliding by the poke center after that hell say soime stuff and go back to return the item he grabbed after that you can go get Heatran. next one cressila (note cressliea and mespirite will have them chase you so be prepared with a bronzong that has a quick clawn and that knows block) youll find that cressilia after the sinnoh dex the sailor that took you to "iron island" his kid will now become sick which you have to go to an island too see cresslia which will drop an item that will make the kid feel better so you grab it and go back to canalave city give the kid the wing and then your ready to go track down cressellia. the rest like deoxys celebi can only be obtained by action replays now sience the events are over. you get a mapahy egg by pokemon ranger its kind of unique.
The legendary is palkia
It's not a legendary in Pokémon Pearl.
on Pokemon pearl you only see 1
the legendary Pokemon in diamond and pearl are dialga and palkia.
No, but there is a Rotom in the TV.
Palkia
you find the legendary Pokemon heatran
The dragon pokemon palkia.
Empoleon is not a legendary Pokemon. It is the final evolution of Piplup, one of the starter Pokemon in Pokemon Diamond, Pearl, and Platinum.
Stark Mountain
No, because it is a legendary.
On Pokemon pearl there are multiple legendary Pokemon. The first true legendary is probbaly palkia. Rotom and spiritomb etc are just sub legendarys. You can also get heatran giratina shaymin and arceus in Pokemon pearl. references,ive finished the game 10 times aswell as platinum and diamond.