Maybe what lavenderblue is saying is that, whilst the literal Greek -> English translation is correct, the phrase/sentence "I want Greek" doesn't really make sense in English. I expected the meaning to be more like "I want to speak/learn Greek", or "I want a Greek one".
This sentence really doesn't make sense. In English you wouldn't say, "I want French." French food? French fries? French pastry? It needs an object like "θέλω ελληνικό φαγητό" = I want Greek food.