If you take an English “FREEVIEW” box to spain it will pick up the free Spanish digital channels through your tv aerial.
Providing of course that you can receive a good enough signal.
Where I am the terrestrial signal is poor and I even had to fit a amplifier to my aerial to receive the main Spanish analog channels TVE, A3 & Tele cinco etc.
But when I tried with a free view box, these channels kept breaking up on the digital signal, and other channels such as La sexta would not open at all.
However if you mean using a free to view “sky” digi box” These really are only any use for Sky due to the very restricted symbol rates embeded in them.
You would be better off buying a cheap free to air satellite receiver (not sky) pointing a dish at either 19.2 east or 30 west and you will get loads of free Spanish channels.