The following is a sample from a current roleplay in which my character is a shiny Eevee. There are a couple of things to note about this sample. The first is that I edited it to remove the names that my partner had given both of our characters. I did that simply as a courtesy to my partner. The second is that it shows how I normally portray my pokémon character’s communication. What you see in quotes is what the human character hears and what you see in italics is what she is actually saying. The better the human knows the pokémon, the better he should be able to understand her. Another pokémon can be presumed to hear the actual words.
Eevee stared in wonder as the the bird disappeared from her view, but she would never forget its majestic appearance. “Veee? Veee. Eeeveee,” she cooed when her trainer picked up the feather and examined it. I don’t know but it’s pretty. Maybe you should keep it. Eevee couldn't grow wings, but she could dream of soaring through the sky, even if it might never be realized.
She calmly rested on his shoulder, but perked her ears again when he spoke of his mother and asked if she wanted to meet her. “Eevee,” she answered quickly, wagging her tail as she did. I would love to. Her stomach growled at the mention of food. She was certainly hungry as her last meal had been the night before, and that was only some random berries she had found. Finally, he asked her if she wanted to test out her skills. She thought herself a pretty good scrapper, having driven off or escaped from more pokemon trainers than she count, and she still never understood why they were always so interested in her. Was she really that special? Still, she was curious about how her skills would measure up in the eyes of her new trainer.
All of that was forgotten when she heard water gurgling just beyond some bushes. Slowly and carefully, she poised herself and jumped from her trainer’s shoulder to the ground. She was much less clumsy this time, and she landed softly on her feet, then bounded around the bushes where she found a small stream. It wasn’t much, and the water was cold, but it was still water. Although it was narrow and shallow to a human, it was deep enough for a small eevee to swim around in, and this eevee had always loved to swim. “Veee! Eeeeveee!” she called out excitedly. Come see this! She swam to the opposite bank, then ran along the water’s edge, splashing as she did. She held her ears high and wagged her tail as she played. Maybe her trainer would join her.