demm Because not only the part "es", the locals with are many, and that way everyone could have in Spanish not only "es_ES" because it is true there is no translation es_PE which is used in the system for date formats, numbers, telephones, currency, but not for translations, if only the part "es" is used it would cover all the Spanish language for those translations.
Or if there is no specific country (in this case es_PE), use the "es_ES", but do not use English