Do they mean "Duolingo"? ("Dewi lingo")
It's a pun on the name of the website and the Welsh name Dewi.
Any differences between 'shwmae', 's'mae', and 'hwyl'? Are they different meanings of saying 'goodbye'?
Shwmae is the south walian version of S'mae and both mean hello. Hwyl is one way of saying goodbye.
I'm hearing an "s" sound before Dewi. Is this the correct way to say Dewi? (Sdewi?)
No, it should be pronounced dewi without an S sound.