Hmm.. just want to double-check that in Greek we'd always say αποτέλεσμα rather than αποτελέσματα? In English, 'a result' would imply that we're talking about one specific effect of the diet - not the overall outcome of the diet.
But in Greek '(ένα) αποτέλεσμα' can refer to the overall outcome of a procedure or event?
Hm, in Br English, I think we could definitely use 'a result' in the singular to refer to the overall result/efficiency of the diet, not just one particular aspect of it. There would be an inferred ["good"] in the sentence, but the singular sounds fine to me.
'Results' sounds a little like Am English to my ears - I think if I heard it, I'd assume that the diet had results or side effects other than the primary intended one of losing weight or improving health.
If I wanted to focus on one particular aspect of the diet, rather than the overall outcome, I'd probably use 'one' rather than 'a'.
This is all a moot point because as a fitness pro, I can say unequivocally, "diets" don't actually get any long term sustainable results - they suck! ;-)
However, back to learning languages......
'The diet has results' does not make sense in English. "the diet gets results" (marked as incorrect) is a better sentence, referring to a specific type of diet.
"dieting gets results" or "will bring results".