Yes. There are many ways of forming plurals in Welsh - 20-30 at a guess, depending on how you count them.
But Welsh pronunciation is so regular that there has to be something to add a bit of complexity and interest to the language!
It's not like English, where we can challenge learners with half a dozen or so ways of pronouncing '-ough-' and two each of '-th-' and '-f-' and '-c-' and '-g-'...
Why doesn't this word for 'countries' have the mutation to take away the 'g' sound after the article?