This isn't crucial, as we can make sure to pass es-419 as the language code in email links which is the bulk of traffic. But I don't know how much or even if that code is used in browser settings e.g. for the people coming from sidebar links. So it would be nice if donatewiki could be somewhat smart about this.
See also pt / pt-br
I wonder if hacking MediaWiki:Lang is one way to do this?