Well, I am spanish and I hope they dont localise the game... I dont care about subs because usually they give you the option to select the language (please do). But if the game voices are in spanish, usually that will mean it will not be in english, due to the amount of space consumed.
I hated that of Skyrim. The spanish voice acting was terrible. Imagine, while english version had 50 voice actors (just inventing numbers), the spanish version had 8 voice actors (again inventing numbers). Everyone sounded the same, it really broke the inmersion of the game.
I do not think they should use spanish voice acting, it will not be enough benefit for their investment. Every person I know (I am from Spain) that plays videogames do speaks english and has no problem at all with english videogames. And some of them (myself included) will prefer the english version of the game over a translation.
Translations are always half-done, are never as good as the original.
So if Bioware invests in using spanish voices, 50% of my circle of friends would simply not care at all, and 50% would look for an english version elsewhere.
However, I can understand about south america. A LOT OF PEOPLE there, even if they do not have much % of roleplaying gamers. So maybe that would be a good enough reason. But for Spain alone? Hell no...