What do you see?

I see "I want an apple" which seems like a fine translation for "Dw i eisiau afal".

And that should be accepted by Duolingo.

It usually writes eisiau (except in a couple of examples when showing dialectal variety) but generally accepts isio. If it doesn't, report it so that it can be added.

WAIT nevermind. Looked at the tips and notes and realized the direct translation would in fact be "I am wanting an apple."

There is a definite article (which looks like yr, 'r, y depending on the surrounding sounds) but no indefinite article.

See the tips and notes for the "Greetings 2" skill https://www.duolingo.com/skill/cy/Greeting2 where the lack of indefinite article is mentioned.

(If you're not reading the tips and notes, you're really missing out. They're not available in the apps, unfortunately, and it seems that there's a test going on whereby some users do not even see them when they open a skill in a browser, which is even more unfortunate.)