TLDR; We are a site about the Spanish Language, run on an English infrastructure. This leads to confusing contradictions, and no clear-cut answer for which language ought to be used.
Questions and answers on the main site may be in either English or Spanish, although Spanish is preferred. Discussions on meta ought to be in English. See the relevant section of help:
Should I post in Spanish or English?
Spanish is preferred, although both languages are accepted. If you are learning Spanish, we suggest that you learn by doing, and ask in Spanish, but there is no obligation. Posts made in English will likely be translated to Spanish for you. When answering a question, we encourage you to answer in the language of the question, if you are able. Don't worry about making a few mistakes, other community members can help with corrections if you are not as strong in that language.
All tags should be in Spanish, except when there is no Spanish equivalent for a concept, then English is accepted.
This text used to be made available in both English and Spanish, until the underlying SE software was re-organized, and the Spanish version was removed for conformity sake.
There is an effort underway to add internationalization and localization to the SE network. Once this is complete, the entire site ought to be available in Spanish, and then I suspect it will be appropriate to convert Meta to Spanish as well.