Translation:Are they on a diet?
"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".
I think it probably depends on your location. Here in South Africa we basically use so-called U.K. English, and to say someone is on diet is the common form. Perhaps our cousins over the pond say 'on a diet'.
On a diet is used here in the UK you cannot say on diet as it would sound incorrect .
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.
So should 'on diet', notwithstanding taalleren's comment. As I pointed out earlier, 'on diet' is perfectly acceptable in most places.