I don't see why we should treat German questions different than English questions, so I will answer the more general question "What language should answers be written in?".
As a rule of thumb, answers should use the same language as the question. This is how we normally would act in normal conversations, too. It also makes the whole page, which can be considered to explore a certain topic, more homogenuous and so easier to read for everyone. E.g. it is easier to track usage of a certain term when questions and answers use the same word for it. IMO this will also make the page easier to index for search engines, leading to higher ranks in result pages. Note that a considerable amount of visitors come here via search engine hits.
But no rule without exceptions. So consider to use the other language
- when you think you could then write a better, more precise, more complete answer. The better your answer is, the better it is for the site in general. In particular it is better to write an answer in the other language than to not answer at all.
- when the language skills needed to read your answer are much more demanding than the level of the question, and you have reason to believe that the OP understands the other language better. For a possible example of this case see ...zum Verhältnis und im Verhältnis zum Verhältnis (according to his/her profile the OP is located in Toronto, Canada and hence is likely to understand English)
If, after considering all this, you are still in doubt, write a German answer. After all, this is German Language SE.