When someone who is clearly a total beginner in his target language, like doesn't even know very basic grammar rules or words and can't even read the script (e.g. Thai, Chinese, Arabic), which language do you use to write answers to this user? Do you use his target language, even though he most possibly can't read the answer, do you use English (if it isn't the target language) or do you use the mother tongue of the other user (if you happen to know it)?
I see this sometimes, that a user is asking a very basic question and some other users write a reply that is more appropriate for intermediate or advanced learners and I wonder how useful this is for the learner (especially if he can't even read the script).
I really try to stay in the target language unless I'm giving complicated or detailed instructions and it would take twice as long to insist on staying in the target language. There is plenty of research to support communicative language teaching, and simple directions should be given in the target language I think. I remembered numbers so much better because my Spanish and Italian teachers wouldn't say "turn to page five" but instead gave this simple instruction in the target language enough times that people recognized it.
For communicative language teaching to work well, it takes a skilled teacher who knows how to rephrase, get the point across with gestures or facial expressions, and it also takes a keen learner who really wants to internalize the language and not just know the rules. A lot of people are learning the language for exams and have very inflexible ideas on how to learn language - usually not based on any peer-reviewed research, but on popular myths about language learning and how they were taught themselves - so aren't open to this method.
It can take extra effort and I have found myself frustrated when I couldn't immediately understand a teacher, but looking back, I learned so much more. That's why sometimes I prefer to work with teachers who don't speak any English because there's no temptation to resort to it on their part, and I get a lot more input when they rephrase and find synonyms.
I came to communicative language teaching very skeptical because I don't believe the sole purpose of language is communication and I like "old-school" teachers. However, I also value research, especially when it's been tested in the classroom, and I think it's a really effective way to learn language if it's done properly.
Yes, it does happen. I especially see this in the Chinese section, that users who clearly don't know the very basics of the Chinese language and therefore most probably can't read Chinese characters at all get lengthy replies in Chinese. I also know another user who learns Farsi just by memorizing sentences, but doesn't learn grammar and can't read Arabic letters. So, how could this user possibly understand a reply written in Farsi?
What I do, is to reply in English, if the question is asked in English and reply in the target language, if the question is asked in the target language. When I do corrections, I also normally write my explanations in German (I only correct German entries). But when I can see from the notebook entry, that the user is still at the total beginner level, I might explain in English (or the mother tongue of the user if I happen to know it) to make sure that the other user understands my explanation.