If you heard it in a listening exercise it would be acceptable as ist and isst sound identical in German. The fact that something is nonsense hasn't stopped Duolingo from using it as a sentence before!
But anyway, context is important in any language. Otherwise there'd be a lot more people panicking about photographers shooting models.
The German word Orange was taken from French and uses the French g sound which is actually somewhat softer than our j. German and French have a long history of interaction and you will hear decidedly French sounds in some words. French has also imported words and word parts from German, but it doesn't really affect their pronunciation. The German g is generally hard, although placement within a word effects the exact sound.
Well, there are two ways you can know. First most German nouns change somewhat in the plural. This is especially true with feminine nouns since the definite article doesn't change. Here it is die Orange singular and die Orangen plural.
The second reason is that German is quite like English in its use of the definite and indefinite article. In English, you can say the child eats oranges but it is not grammatically correct to say the child eats orange. You would either say an orange or the orange. The same is true in German.