How do you say it when it has no defined gender?
For example in the phrase:
Is the French translation “mangez-la ! ” or “mangez-le ! ”, or neither?
Also, aside from in “c'est” where is “ce” used?
You could use Mangez ça!, although it's more a translation of Eat that!. Otherwise, by default you might use the masculine Mangez-le!.
EDIT : I made a wrong assumption, thanks Stéphane for the heads-up
Mange-le ou Mange-la will most probably get the result you want.
The person will understand Mange le fruit ou Mange la pomme with the same end result.
In french every name has a gender, which is a mandatory syntactic attribute, not a matter of biology. We use it to know to which name relates an adjective (with an efficiency of 75%). As 50% of our vocabulary is masculine and 50% is feminine, you will always find a word with the appropriate gender describing your it.
In facts, if your it has no definite gender, it can only be amour, délice, orgue, gens, oeuvre, alvéole, après-midi, autoroute, enzyme, réglisse,... But even then, they take a gender as soon as they enter in a phrase.
About the distinction between grammatical and natural gender, did you know that according to wikipedia, in old English wif (wife) was neutral and wifmann (woman) was masculine ?
I would tend to say that your question is based on a false assumption: "it has no defined gender" - this situation should never happen in French at all as all nouns have a definite gender.
But yeah, in the case you don't want to name the thing or can't find a noun for it you can use "Mange ça!", or even simply "Mange!" if there's nothing else to eat. I'm deliberatley suggesting "Mange" instead of "Mangez" since I feel there isn't much respect involved at this point, but that would of course depend on the context.