SPA means the language of origin of the game is Spanish. When starting Pokémon XY, you can choose from seven different languages for the game to be in. These are Japanese, English, French, Italian, German, Spanish, and Korean. Pokémon who originate from any language have that indicated, like SPA for Spanish or ENG for English. Despite what the other answers may say, it doesn't necessarily mean they're exclusively from languages these countries are from, as there are very well people who speak these languages outside a country they originate from.
Pokémon traded with a language the recipient is not playing on leads to a few unique things. For instance, if they're not nicknamed, they'll retain the name of the Pokémon from that country. The Pokedex entry for the Pokémon is also gotten in its country of origin. Finally, the Pokémon itself gains a 1.7× experience modifier instead of the standard 1.5× for traded Pokémon, and it can give greater Shiny odds when breeding via the Masuda Method.