Germany: revolving credit is the expensive way to do this
The first thing to say about a German card balance is that most people do not have one. The standard German card is a charge card: the full balance is collected by direct debit from the current account once a month, and no interest arises at all. Where a revolving option exists it is usually opt-in, and the calculator's minimum-payment mode only describes what happens once it is switched on.
That matters because Germany has a much cheaper alternative sitting next to it. A Ratenkredit, a fixed instalment loan from a bank, is the normal way to carry a large purchase here, and it is typically priced several points below revolving card credit with a fixed term and a fixed payment. Anyone modelling a German card balance over more than a few months is usually modelling the wrong product; the loan calculator is the better tool.
Germany sets no statutory interest cap. Courts instead treat a rate roughly double the market average, or more than about twelve points above it, as usurious under section 138 of the Burgerliches Gesetzbuch, which voids the contract. It is a soft ceiling and it moves with the market.
Revolving credit here typically requires about 3% of the balance a month, with the interest taken from within that payment and a floor of about €25.