Animals are generally not capable of learning and using human language for several reasons:

Brain structure: Humans have specialised brain regions, like Broca’s areas and Wernicke’s areas, dedicated to language processing and production. Most animals lack these specialised language centres.

Vocal anatomy: Most animals don’t have the physical structures required for producing the range of sounds used in human speech. For example, most animals lack the precise control over their vocal cords, tongues, and lips needed to articulate human words.

Lack of evolutionary need: Animals have not evolved the need for complex language systems like humans have. Their communication needs are generally met through simpler vocalizations, body language, and sent markers.

Inability to combine symbols: A key feature of human language is the ability to combine symbols (words) in new ways to create new meanings. Most animals lack this ability to use language in such a way. 

Most researchers believe that animals lack the cognitive capacity to process the complexity of human language. In recent years, we learned more about advanced cognition in animals. We have produced a podcast “What Can AI Learn From Studies Of Animal Cognition? With Ludwig Huber” (“AI Snacks With Romy&Roby,” episode 9) and invite you to listen in for more information.

In the “Romy and Roby” series, all animals speak human language. This is a science-fiction element along with Roby possessing general artificial or human-like intelligence. Everything else in the books is rooted in the existing technologies and science.