All the interviews I've seen from SE about spanish language added to FFXIV, say yes in the future, but cite difficulty in dialect/colloquial translations. I mean most other languages have colloquial differences, but Spanish has a ton, and its one of the most widely spoken languages, where other languages with huge colloquial differences may not be as widely spoken. Its just a lot more effort on their localization team than it is for any other languages they've done that's for sure.