I think we need to allow both.
I have two reasons for this:
- Many beginners to the Spanish language won't be able to participate if they are required to read/write everything in Spanish.
- Many beginners to the English language won't be able to partcipate if they are required to read/write everything in English.
I would like to encourage non-English speakers to use this site (and other SE sites), as I thik that will make this site stronger, as well as make the rest of SE stronger (especially as other bi/multi-lingual sites are being launched).
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.
If we do choose to accept Q&A in both languages, that does mean we may find duplicates. When this happens, I would encourage closing/merging them, and translating the relevant Q/A into both languages so that both OPs can benefit.