So, I had to go on a bit of a bulbapedia chain.
First, I went to the article about a random dude, brock, hoping to find some spanish dialogue, which I didn't see latin american spanish, and I didn't see any option for castillian. Undeterred, I decided to check out the article "Pokemon in Spain". This didn't tell me much, so I decided to check out "Pokemon for Latin America". This lead me to a very interesting development.
All of the core series Pokémon games have been released in Latin American countries. Most games are available in English only, imported from the United States; however, Pokémon Red and Blue were also available in Spanish, using the translation from Spain. The Spanish versions of Red and Blue were not available until 1999, months after the original release in English in October 1998, and were re-released on the Virtual Console alongside the English and French versions. No other Pokémon games were available in Spanish in Latin America until the release of Pokémon HeartGold and SoulSilver, which also used the Spanish translation from Spain. Despite the content of the games being almost identical to that of the releases from Spain, the Latin American Spanish versions of the games have their own country code in the serial numbers, LTN (instead of USA for English North America, or ESP for Spain, etc.). As Pokémon X and Y feature multilingual support, the need for separate English and Spanish Game Cards from Generation VI onwards appears to have been eliminated, as players may choose their preferred language at the beginning of the game.
This shows us that the important information, which is the translation, is being transferred from the Spanish mainland, presumably because Castilian is the "original", or "parent" form (By parent, I do not mean to demean a language, it is a term used to describe the language of which dialects spring off of). This shows us that it uses Castillian, or that's what I believe anyways. I abused bulbapedia for this, I hope it's good enough.