Language is shown for fixing name values, and is derived from matching the current Nickname string against any language. English is the first one it matched.
Spanish pkm names are the same as English ones, with the exception of Egg (and Type: Null in Gen7). I'll add it in as a permitted gen1/2 language ID (Egg name being different was overlooked).
https://github.com/kwsch/PKHeX/commit/b5640de538cd54b58edf6cb8c79230159b90795c