This may best be answered by considering the various audiences who use the site:
1) Novice students of German, from many countries, who might only share basic German or English as a lingua franca.
2) Intermediate/Advanced non-native speakers of German seeking to improve their proficiency.
3) Native German speakers who are younger or still in school themselves(?)
4) Advanced native speakers more likely to answer than to ask questions.
People in all 4 groups:
- benefit from responses in German
- benefit from responses they can fully understand
People in Group 1 are most likely to:
- ask questions in English (even if it's not their 1st language)
- misunderstand German-only responses & need additional English explanations
People at the "Intermediate" end of Category 2 might:
- not fully/properly understand responses given only in German & may require English clarification
So, in a perfect world, every response would be auf Deutsch
Warum nicht? Even if the person who asked in English doesn't currently understand it all, it helps them improve and adds to the repository of German text available to people at a similar proficiency level, or those seeking related answers.
This also ensures that more answers remain accessible to German speakers of all levels who do not read English (whether Chinese, Russian, Turkish, Arab, or even younger Germans).
BUT, questions asked in English should ALSO be answered in English.
These are the people least likely to understand a German-only response.
If they can't understand your answer, what's the point? You've just generated more questions.
Answering English w/English (in addition to German) also assists other novice German learners who may be more familiar w/English than German, even if it's not their native tongue.
What about questions asked in German?
For novice German learners, if they don't understand the question itself the response language is irrelevant. If they get part of it, the responses help improve their reading & vocab. The other 3 audiences likely prefer German responses anyway.
For "Intermediate" German speakers, perhaps it's best to place the burden on the person asking to request clarification in English if necessary? I've often run into questions/responses that I have a hard time fully understanding, and would benefit from English/bilingual explanation ... At the same time, I appreciate the challenge of having to first try German & then fill in the gaps, rather than have a tantalizing English translation handy to enable my lazy tendencies.
Can/will everyone who answers questions be perfectly bilingual & take the time to write everything twice? Probably not, so this is kind of moot ... People are going to write in whichever language they're most comfortable with.