What language do you want your answer in?
I've seen this happen many times, twice today. Someone asks a question and many people respond with the answer in the wrong language. It would help if the question specified what language the answer should be in, e.g., How do you say (something) in English/Spanish?
On the other hand, responders could check the questioner's profile to see what their native and target languages are. I know not everyone fills out this part, but many do.
Just a suggestion.
10 Answers
Golden Rule:
Answer in the same language it was written in. If you must write it in another language, use basic stuff (if the question is basic), and more advanced stuff (if the question seems more advanced), because most people ask questions based on what level they are on. It is good practice for the one who is asking to read in their non-native language.
That's my opinion on it. Others may think differently, so I guess we'll see.
I think this is a difficult question. I think it is fair to expect that when answering that one go to the persons profile page first- it's not hard to do. I think it is fair to answer in any language that the person rates intermediate or above. It is a little harder when one rates oneself at a beginner level (as I do). I personally have no issue being answered in Spanish if it is not too complex- I can usually even work through tenses I haven't yet studied, and learn from it. I actually enjoy getting a mix of answers in both languages.
I did, a few days ago, see an answer (from a native Spanish speaker) to someone who listed themselves as fluent in Spanish, beginner in English, for the question:
Comó puedo decir "Buenas Tardes" en inglés?
That answer was a stock answer explaining how the translation tool at the site works, in fairly complex English. The next answer was from a native English speaker also in English, that answered the question- but I am not sure that someone asking that question was ready for an answer in English.
Since no one else seemed to be willing to answer in Spanish, I did my best (although probably with incorrect Spanish, hopefully at least comprehensible), but honestly, to properly answer a question, I think I still need a level of Spanish higher than mine, and I generally try to avoid questions that I think should be answered in Spanish, keeping my Spanish to where I can use examples that are at my level, or language learning exercises where it is okay if I get it wrong.
I think for anyone above beginner in both languages, the answer could be in any language that the answerer is comfortable answering in.
Maybe I'm not being clear this time. What I mean is that not everyone here is trying to learn Spanish. Some are native Spanish speakers who are also very good at English, but are asking for the answer to a problem in English. If someone says, "What is the past tense of (verb)?", they may not be asking for the past tense of the Spanish verb, they may want to know the past tense of the English verb. Same thing with, "What is the synonym of (word)?". They may really want the English synonym, not the Spanish synonym.
Because I'm trying to practice Spanish I often ask questions in Spanish, even though I'm usually asking for the meaning of a Spanish word or phrase. Everyone assumes (correctly) that I want the meaning of the Spanish word or phrase. On the other hand, many people ask questions in English and people still assume they want the meaning of the English word or phrase translated into Spanish. That's not always the case.
What language do you want your answer in?
I would like that all my questions be answered entirely in Spanish. Thanks 
Japanese por favor.
That has happened to me...
I see the question, and I think of the answer... usually I write in English, and when I realize that the poster wanted the answer in Spanish I edit my answer and put Spanish below the English one...
Normally I check the profile... but that's after I answer... and yeah, I sometimes feel that I should answer in that guy's language... but this being a English-Spanish site has stopped me... jaja
I once answered in Italian...
I have noticed that occasionally both questions and answers mix Spanish and English in the same paragraph. Another recent question had a headline in one language but the full text of the question in the other. I like this for two reasons. It lets me know I have company at my level of foreign language development and it makes me think the answer is expressed as well as possible. (Two languages are better than one!)
I don't think there is a wrong language on this site. Because of our two languages, we are reading our answers either in the language we are more comfortable with or the one we are working hard to learn anyway. The machine translations couldn't be any closer, either.
On another post KevinB suggested using Psst, I found that language unacceptable
and seeing that we are unable to shout on SD I could not ot get my message across. 
¿Qué significa 'gato'? Gato. Oquey.
- KevinB
También se puede decir que un gato es un mamífero carnívoro de la familia de los Félidos. Un animal doméstico, el gato tiene cabeza redonda, lengua muy áspera, cuatro patas cortas y pelaje espeso, suave, de color blanco, gris, pardo, rojizo o negro. Es muy útil en las casas como cazador de ratones. Y en algunos casos (muy raros), pueden aprender a tocar el banjo como Pete Seeger.
Y si no soy capaz de comprender las palabras.........envíame una foto
Gato?
spanish because its easier
