You can only change how people see you. There is a psychic in the Pokemon center of violet city (second floor) who will let you chose who you want to be. You can choose between 4 kinds of trainers.
Chat with our AI personalities
No. You can't change languages. Even in SoulSilver.
You can't, as with all traded Pokemon, you can't change their name.
To change your appearance in Pokemon Platinum's Union Room you need to talk to an NPC in the Oreburgh Pokemon Center. The NPC in question has the appearance of a Psychic trainer.
man in violet city Pokemon center
no you can trade soulsilver pokemon to black