It's not getting word pronunciations slightly wrong, it's talking in a made up language. My sis and I did it when older on car journeys and called it 'Funny French'.
It doesn't feel like 'regression' but just wondered if other people's children did this. He laughs when he does it so it seems like a play thing.
In general DS has wide vocab and pretty good pronunciation; he's beginning to put together 3-4 word sentences.