Whereas using ISO 639 would be easy (easier?), straightforward, explicit as using only intrisic information (available on the item itself), and reduce significantly the length of such entries, simplifying [AR][BO][CL][CO][CR][EC][SV][GT][HN][MX][NI][PA][PE][VE][…] with [ES], while actually making this sole information both correct and complete in what distinguish such items from the “default” ones (generally US for NA, UK for Europe, etc.) of the same region.
Coming from a Spanish speaking country, it can't be realistically applied. [ES] refers specifically to Spain (es-ES), and even thought they're mostly interchangeable, games that come in Castilian Spanish (es-ES) tend not to do well in Latin America (es-419), and vice-versa.
Metal Gear Solid was never oficially released in Spanish for Latin America, because the only dub was made for Spain, with a very thick Castilian accent, while the Spanish release of
Halo 2, with a
very Mexican dub, turned into a laughing stock in Spain.
There's also the big line of code of countries in ISO 639, but that can't be realistically replaced for the
UN M49 codes, because Mexico and Brazil use different rating standards.
Spanish-speaking region releases are a big pain to adapt to the way the website works now.