The ersatz version of something is an inferior substitute or imitation. You might speak in an ersatz French accent, but you won’t fool the Parisian who runs the local French bakery.
Used as a noun or adjective, ersatz stems from the German verb ersetzen, which means “to replace.” Compared with regular Coke, Diet Coke may be considered an ersatz beverage because of its artificial sweeteners. Similarly, rich, creamy, full-fat ice cream is much better than the ersatz, fat-free version, or frozen yogurt. You would be in deep trouble if your fiancé found out that her engagement ring was an ersatz diamond.