Seamos honestos (Lets be honest)... Spain is not the biggest market for MMOs... or videogames in general... Germany, France and the UK are the top 3 consumers of videogames in europe, so yeah, its normal that the game is in french and german before spanish.
That said, Spain is the fourth or maybe fifth (consumer in Europe) depending on how the market has grown or decrease this year, which in my opinion, its still an important number of players.
Taking in factor that spanish is one of the most spoken languages in the world, i think that adding subtlitles could spread their market and gain more players (in Spain and South A.).
But translating the WHOLE game to Spanish... thats too much, mainly because the spanish from America and the one from Europe are different (believe it or not...).
Another thing to add, all spanish voice actors are HORRIBLE!
TL;DR : Spanish subtitles would have been nice, hope to see them in the future.