This is a little trickier than it seems at first. If we just check to see whether we have a value for it at all, we'll be hiding the fields on form re-display (for example, when an error occurs on the first form post). We'll have to implement a thing we've thought about for a while, which is to track in session WHERE we found each piece of data.
This is deployed - the radio buttons for the opt_in choices only appear when the donor is from a country where the field is required, and the donor has NOT indicated an opt_in choice on a banner or donatewiki.
However, the only countries where the field is required are IE and GB - @CCogdill_WMF and others, should we add more countries to that list?
Translations exist for the yes/no text and the explanation below in these languages:
Besides GB & IE for English, it looks like we can add Germany, France, Portugal, Ukraine, and North Macedonia. I guess Asturian (ast) is spoken in a part of Spain and Tarantino (roa-tara) in a part of Italy, but since es and it aren't on the list yet we shouldn't jump the gun.
Ah, cool, we can add those directly in the DonationInterface code, or via Translatewiki here: https://translatewiki.net/w/i.php?title=Special:Translate&group=ext-di-gc&language=it&filter=%21translated&optional=1&action=translate