Foreign Currency Mortgage is a mortgage with the added risk that the loan is received in a foreign currency and must be repaid in the same currency.
The installments are in the currency in which the loan was formalized, so the amount to be repaid will depend at each moment on the exchange rate to be applied. The outstanding debt will depend not only on the amount of capital that we have repaid and the interest rates, as occurs in the rest of the mortgages, but also on the quotation of the currency in question, that is to say, on the variation of the exchange rates.
This is a high-risk product, in which the debt can increase stratospherically precisely because the amount to be repaid will depend on the variation of exchange rates in the currency market.