By not giving them any rights even though complex expression through language implies a high level of intelligence and sapience. The dog and cat that you can still talk to are still considered your property that you can just take to the vet and put down. Animal shelters can now just tell the animals their kill dates so they don’t get too hopeful. Slaughterhouses will be filled with screams you understand instead of just screams. Cosmetic companies and labs can now ask their test subjects for the level of pain while smearing makeup in their eyes. Sea World, zoos, circuses, etc would make peak profits now that they can address their slaves like they’re human slaves.

I mean, all humans can communicate with each other and we still don’t give rights to some of them. Anyone who thinks we’ll be nice to animals just because they can talk is either a child or otherwise doesn’t know how humans operate.