An interesting split in the world lines. I'm going to be interested in seeing how this one developes. It's not as drastic a split as some stories, but it's clear that Albus Dumbledore is in for some difficult times as are the Dursleys (I can't say that I really weap for either). As to felines having their own language, consider Diane Duane's "Feline Wizards" series (which I most highly recommend), "The Book of Night With Moon" and "To See the Queen", and the feline language there.
This looks to be a most interesting read that I shall follow.