Alex had received augmentation to his brain from Doctor Mor'phas a Klingon mad scientist. This renders him a sentient individual, although at the root level he is fully a cat. Alex can communicate in Federation Standard, if he feels like it. While his thoughts and desires are those of a cat, he is better equipped to tell you what they are.
Alex also has a cybernetic layer under his skin that renders him invisible to scanners. He enjoys this.