I've always just considered it translated animal language. But it seems like a lot of people think of it as them talking.
Let's take the Rugrats for example..
They don't speak English. They speak baby, yet we understand them.
Or, Garfield and Heatcliff. Neither speak human english, it's translated animal talk.
Or actually, most cases of animals talking in fiction.
Or, within Disney itself, look a other films. Like Aladdin, I doubt they're speaking English (or whatever language the dub you're hearing has them as). They just translate it for convenience.
Do you really want to watch a movie about bunch of animals "speaking" their native animal language, with subtitles?