I'd need to know how the Pokemon speak. I mean, the Pokemon from long ago, I know they say their species names. I want to know how the Pokemon in my sig would speak.
And do they say other words? Id not, then I guess I can give them different languages.
When you say "Pokemon from long ago," do you mean that your story takes place in the distant past? That can make a big difference.
But yes, traditionally they do just say their names or give similar feral cries. That doesn't change much between each species.
The anime has them saying their names. Games have their cries, like real animals. PMD has them speaking human speech.
While the first two instances are true, I think the Mystery Dungeon games see them using English because it's translated Poke-speak. (I mean it would be pretty difficult to understand and enjoy the games if you can't understand what's being said between the characters.)
For that reason, I think it's fair that if there are no humans in your story, you can translate all the Poke-talk into English so that readers will know what's going on. However, if say one character speaks in a strange, different way that most of the other characters don't understand, you can have this reflected in the speech by showing their speech as being harder to understand. They can speak Spanish (which some people/Pokemon might understand) or gibberish or just communicate in a series of dances and clicks to show the barrier in language between them.
However, if there
are humans...well, when humans hear Pokemon speaking, all they hear is their name being said over and over. For that reason, I think it works best for your human readers to read the same thing. Since people
can understand Pokemon to some degree, however, you can often give the reader an idea of what the Pokemon is saying based on how they say it and how the humans react.
Alternatively, you
can just translate all the Pokemon talk, but canon says that humans don't understand Pokemon perfectly, and it can be weird when that suddenly changes in a fic. Personally I find it kind of cheap to do, but some readers find it a lot more helpful to understand every word a Pokemon is saying.
Either way, keep one thing in mind:
Pokemon are not people. They don't act like humans and they don't talk like humans. If they do, then they're not so much Pokemon as they are people in Pokemon bodies. Seeing a Pokemon acting and speaking the way a wild animal does can add a lot of spice to your story.
Sorry for the long rant, but I hope it helps!
~Psychic