@RandyHayes
Not sure if it is affecting or not, but my internet connection and the PowerApps enviroment are both from the US, from the place I work at. I did try using [es-419] which I think was the correct one for my case, but didn't work.
Also, it is a detail form, I unlocked the datacard I wanted to change, and added the Text( xx, "dd/yy/mmmm") on it, with xx=Parent.Default; on the Text property of the DataCardValue. I assume by the symbol in the list of screens it is a label, though.
EDIT:
What worked was to do the "text" thing on the datacard itself, on the Default property, thus ending as:
Default: Text(ThisItem.'Comienzo Plan', "[$-en-US]dd/mm/yyyy]"
And I know it worked because it also does not show the HH:MM it had as default (without changes, I mean)