I entered 'Don't talk to Gareth about the car' - which Duolingo didn't like. It suggested two options, one of which was utter drivel ... 'Don't say to Gareth about the car'. A total nonsense sentence! Don't say WHAT to Gareth about the car. A poor translation.
"Don't talk to Gareth about the car" would be ""Peidiwch (â) siarad efo/gyda/â Gareth am y car".
As an English speaking valley boy, "Don't say to Gareth about the car" is a sentence I would have used regularly to warn someone not to say something. The "about" part of the sentence would be known to the two of us, for example, where I had borrowed Gareth's car and taken Dewi Lingo out for a drive, then scratched the car on a gate post. When meeting up with Dewi and Gareth later it would make perfect sense to say to Dewi out of earshot of Gareth, "Don't say to Gareth about the car" . Maybe a valleys English dialect sentence translated directly from the Welsh that was spoken there in the 60's.