Does Croeso mean both 'welcome' as in a way of greeting and 'you're welcome' as in a reply to thank you?
It can mean "You're welcome" as well as "Welcome" (A greeting) :)
what does Dewi mean?
"Dewi Lingo" is being used as the name of a person throughout this course. Dewi is a common Welsh name, and "Dewi Lingo" sounds like Duolingo.
The Dewi Lingo pun wore thin after about the.. first time I saw it. It's a chore to type it out all the time when it's not really what I'm trying to learn.