The language version is meant for all Spanish-speaking countries, not only for Spain. I wonder if the “locale” is wrong in that case? Or what is the correct value here?
I think I’d set the locale to the version of the language variant I’m actually using. Like when I write German, it is German German, not Austrian German, although someone from Austria or Switzerland will probably read that version rather than English. If you use a more neutral Latin American version of Spanish, you could probably use ‘en_LA’.
Google speaks about “country based language variations”. As you do not want to create a specific version for Spain but one for every spanish speaker I would just go for es.
Later you can add a specific version for Mexico and Spain, those would be es_ES and es_MX - you can still keep your generic spanish version for all other spanish speakers. Im your Spanish for Spain you could change the Currency Symbol to €uro and in spanish for Mexico you could change it to Pesos.