I'm currently writing a Pokemon story but i've run into a problem. I don't know how to chat dialog for Pokemon. I know what i'm doing for Pokemon to Pokemon, but Pokemon to trainer I don't know what to do. I'd appreciate it if someone could help me please.
You could not write out the dialogue. That's what I do. Like instead of having "Pika pikachu pi pika chupi pik achu!" I just have "Artemis tugged on his trainer's leg and chatter, furiously pointing at something." I pretty much translate it anyhow (either by having said trainer translate it for other characters or through narration) so it's understandable. And less annoying to read.
On the subject of Pokemon-Human communications, there are a few routes you can take to make it seem as natural as possible, as in its not too jarring for your readers to read. For one, you could go with Astinus' suggestion and make the trainer able to understand the Pokemon based on its actions. Usually, this comes with a close bond between the two, but authors have been known to ignore this.
That leads me to method two. You write out Pokemon dialogue normally in humanspeak with them using their own tongue in-universe and your human characters will somehow understand the gist of what the Pokemon is saying. For example:
"What's that over there?" said the yellow rodent while pointing at a boulder.
"I'm not sure, Pikachu, but there does seem to be someting odd about it," said Ash, contemplating the boulder.
Notice that the character doesn't actually understand the literal translation but can somehow get what the Pokemon is saying. Usually Pokespeak is written in italics to differentiate the two. The only difference between this one and the last is that you're actually letting the reader know what the Pokemon is saying.
The third method is slightly simpler and involves having the trainers understand the Pokemon in-universe without any gimmicks. Either the Pokemon have learnt humanspeak or humans have learnt Pokespeak. Implausible, but useful if you want to be as flexible with your characters as possible.
And that's about it really... There are possibly more ways for human-Pokemon communication, but those are the three that I can remember right now.