I'd be more than happy with the subtitles. Halo Castillian dubs and translations are awful, anyway. I hated that you couldn't change the language in previous generation Halo games, save for anniversary. I've ended up buying both the Spanish and UK editions for most of those games, save for 4; I straight bought the UK version for that one. The Spanish translations are inconsistent and sometimes just ruin the dialogue.
I could give tons of examples of how dreadful they are: botched spaceship names (from Halo 3: "Aquí la Forward hacia la Dawn". WTH?), changing names of the various factions (Pacto-Covenant, Precursores-Forerunners), voice actors changing with every single game, awful delivery in many places, grossly mistranslated lines (Arbiter's "Were it so easy" turns into "Fue muy fácil" ["It was very easy"]). Hell, we even had stupid contests in which a random Halo fan, usually a whiny kid, would dub a few lines from the game; the delivery for those is simply pathetic:
https://www.youtube.com/watch?v=6hTphwM7NoM#t=47s
https://www.youtube.com/watch?v=tVfg291ccbI#t=1m37s
I'd be perfectly fine with not having a Spanish dub. They just range from hilariously bad (Halo 2) to plain bad (Halo: CE) to serviceable (3, ODST, Reach and 4). Not having proper subtitles, though, is inexcusable. This is the first time I see something like that happen from a major publisher.
Also, if they force the Spanish dubs for the other games, I'll be mad. There's no excuse now for not having the English voices. Even Fable did it right last gen by letting users download the original English dub.