The consensus on Should we allow question and answer bodies in Spanish? seems to be that both Spanish and English should be allowed.
However, what about mixing both languages within a single question/anwsers page? For example in the English question: How did the words “mataburros” and “tumbaburros” come to mean “dictionary”?, currently the top voted answer is written in Spanish. As an external observer, this seems a bit odd, and for me it kind of breaks the flow when I'm trying to read the question/answer pair.
Another problem I see is, when a beginner learning Spanish asks a question in English (which was the rationale behind accepting questions in both languages), he might have trouble reading a Spanish answer so, even if it is correct, it isn't of much use to him. And similar for a person not experienced in English, getting English answers on his Spanish question might not be very useful to him.
The only argument I see for allowing mixed Q/A bodies is that, an answer is better than no answer. So if someone knows the answer but is not fluent in the language of the question, it is better to write the answer anyway, instead of not sharing that knowledge. But then perhaps it should be allowed, or even encouraged, for third-parties to edit the answer and provide a translation in the original language of the question?