I addressed part of this question early on in this answer to Should we allow question and answer bodies in Spanish? where I suggest:
I also think we should, when possible, try to respect the language of the question. That is to say, if the question is in English, answer in English, if it's in Spanish, answer in Spanish. However, if you don't feel you speak the OP's language strongly enough to answer in their language, answering in your own language is better than not answering at all. Someone else can always translate if appropriate.
I think that advice still holds (although the rest of that question and answer seem pretty obsolete now).
I would also add that the site seems to be taking a preference for Spanish over English (which I welcome and encourage!), so while I still believe in honoring the question's language, when that language is English, I encourage answering in both English and Spanish.
I also encourage translating questions in English into Spanish (leaving the English version there for reference and the benefit of the OP).
Providing English translations for existing Spanish posts is also welcome, of course, but I personally consider that to be a much lower priority.