The rabbi sez:
As far as I know, the origins of the phrase are not Yiddish; I believe the phrase was originally coined in America. Being that chopped liver was always considered a side dish and not a main course, the phrase is used to express hurt and amazement when a person feels he has been overlooked and treated just like a “side dish.”
Not definitive, of course, but it suggests the earliest use will not have been Yiddish (which might make it easier to find). And I would be very surprised indeed if it existed in other languages.