I've overseen national language support (NLS) in multiple products. For things like publications general translation skills are fine but for text or messages issued from code you typically need someone with enough programming skills to be able translate the message/text files and compile them for verification. You also add onto the product test cycles as each language gets tested independently. Another cost during translation, at least for English, is developer time taken up explaining ambiguous words. Take invalid, one meaning is a disabled person and the other is the opposite of the word valid. There are many other multiple meaning words like this. None of this comes cheap nor can it be done overnight. I will say Spanish was one of the languages we always got asked to support and rarely did we do it. The fact was with limited funds for NLS we had to cover our largest revenue bases. These were usually Arabic, French, German and Japanese. Towards the time I retired Chinese was also hitting critical mass. My guess is SE is in a similar position.

I've also seen some comments as to Latin countries. Many of these countries actually speak Portuguese so translating the game into Spanish still only serves part of the market.

Lastly, which dialect of Spanish? While Spanish speakers can understand each other much like we English speakers there are enough different dialects that sometime you wonder are we all speaking the same language?