What is the difference between es and es-419 in iOS localization? 

es is Spanish without a region: the generic localization that any Spanish-speaking user can receive. es-419 is Spanish for Latin America. The number is not a country code. ISO 3166 has two-letter codes for countries only, so when a region has no such code Apple's language identifiers use the numeric UN M.49 code instead, and 419 is "Latin America and the Caribbean" in that list (see UN M49 list). Foundation shows it as "Spanish (Latin America)".

The two exist because vocabulary and forms of address differ between Spain and the Americas ("ordenador" versus "computadora", "vosotros" versus "ustedes"), while one translation per country would mean twenty localizations that mostly repeat each other. es-419 is the shared variant for all of them.

iOS matches the user's language and region against the localizations in the app bundle, preferring the most specific match and falling back within the same language before trying the next language:

• A user with Spanish (Mexico), es-MX, gets es-419 when the app has it, otherwise es. Users with es-AR, es-CO or es-US land on es-419 the same way.
• A user with Spanish (Spain), es-ES, gets es. If the app ships only es-419, that is what they get, because another variant of the same language still beats a different language.
• A user whose device is set to plain Spanish gets es.

A practical setup is to ship es as the complete, neutral Spanish localization and add es-419 when you want Latin American wording. You rarely need es-MX or es-AR on top of that, unless one market needs its own terminology.

The same pattern applies to other languages. For Portuguese, a pt-BR user gets pt-BR and then pt, and a pt-PT user gets pt. For Chinese, the script is the useful split rather than the region: a zh-TW or zh-HK user gets zh-Hant, and a zh-CN user gets zh-Hans. Language, script and region codes are listed at Language and country codes.