"Are they on diet" should be accepted. It is commonly used in English!
Really? I think "on a diet" sounds more natural, I've never heard just "on diet".
Why is "Zijn zij op een dieet?" not accepted HERE but it was in a translation? <_<
Do you always say "Are you on diet" or can you ever throw the article in there in Dutch for this sentence?
"Op een dieet" should be accepted, yes.