The problem with that is that animals have to have an intelligence of 1 or 2:
Intelligence score of 1 or 2 (no creature with an Intelligence score
of 3 or higher can be an animal).
To speak, you need an intelligence of 3:
If you have a penalty, you can still read and speak your racial
languages unless your Intelligence is lower than 3.
So if you do that, the companion PC is no longer an animal. It's a magical beast, and thus not eligible to be an animal companion.
You could house rule away that restriction of course, but I don't think it's a good idea. If the PC in question wants to play as a magical animal that happens to be the Druid's friend, that would be an easier way to go about it. That lets him play a fully realized character with his own will & ability to interact with the party, instead of what is functionally a trained animal. You could use the monster advancement rules to let him gain levels, or let him take class levels if he wanted to become a Cleric Unicorn or something.