As many people will know, on June the 16th of this year, became effective the new mortgage law (law 5/2019), which regulates the mainly the mortgage contracts and provide much more security to the individuals before the banks and lending insituttions.
Going thorugh the law we find some very interesting articles which I belive can be of the interest of amy people, like art 20
In real estate loan contracts that are denominated in foreign currency the borrower will have the right to convert the loan to an alternative currency in accordance with the provisions of this article. This alternative currency will be:
a) the currency in which the borrower receives most of the income or has the majority of his assets with which he has to repay the loan, as indicated at the time was done the most recent solvency assessment related to the loan contract, or
b) the currency of the State in which the borrower was a resident on the date of conclusion of the loan contract or is a resident at the time the conversion is requested
The exchange rate used in the conversion will be the exchange rate on the date on which the conversion is requested, unless otherwise contractually established. For these purposes, and unless the loan agreement provides otherwise, the exchange rate used for the conversion will be the one published by the European Central Bank on the date on which the conversion is requested.
This means even you initially when you contract your mortgage in Spain, you do it in Euros, but you are resident or you receive the majority of your income in a different currency, you can decide on a determinate moment to convert the mortgage to currency avoinding having to exchange funds every month and as well an even more importat not been subject to the possible fluctuations of the Euro against your currency.