I'm not where I can cite sources, sorry, but I would say sponge anything in this sense is probably [Brit.]; I thought it was like sponge cake, that is, a soft cake similar to (AE) pound cake.
Ladyfingers (so spelled) I know only as a kind of flat, elongated oval cookies, usually store-bought, that are used in certain dessert recipes, as a kind of semi-crust under ice cream, fruit, custard, etc. They're based on plain white flour and are sort of soft and bland, like vanilla wafers without the vanilla flavor.
Biscotti, on the other hand, are hard-baked Italian-style cookies served for dipping in coffee, made by slicing rolled, flattened dough on the baking sheet into long segments. They're double-baked and are very hard when dry, but soften when dipped in coffee; they may contain nuts or chocolate. In the US they've only become popular in the last couple of decades, with the rise of coffee bars and specialized coffee styles such as latte. English speakers unfamiliar with Italian may not realize that one alone is a biscotto.
So the two English terms aren't the same thing at all, but perhaps we could clarify which of them correspond to 'die Biskotte.' Perhaps naka-naka could repost whatever got lost between the angle brackets.