So last time I was talking about the danger of the philosophy of answer-in-sentences, but I mentioned the English-only-classroom in passing.
It's a commonly stated goal that we, as teachers, should be aiming to use only the target language in the classroom, and that we shouldn't be introducing "translation". In the guidelines of my school, translation is marked down as a "last resort".
However, most teachers kid themselves on. In a heterogeneous, mixed-mother-tongue class (foreigners studying in an English-speaking country), obviously you can't use the students' mother tongue as there is no other shared language, but once you go into a homogenous class, translation becomes a matter of course. I've observed other teachers doing it, and I've even had advice from teachers that has thrown in translation as a completely standard tool. (EG. a former boss in Spain who insisted that he taught in English only told me that the best way to correct Spanish kids saying "one car" instead of "a car" was to say "uno coche?" thus demonstrating that "one" was explicitly a number. He was completely unconscious of the dichotomy of philosophy vs practice.)
When I've witnessed this in observed classes, what I tend to experience is a careful, elaborate explanation of a task or grammar point in consciously graded English, followed by a very quick, concise summary in the students' native language. How are we to know whether it's the difficult-to-understand English or the exceptionally easy mother-tongue instruction that they're learning from? Common sense, perhaps? It strikes me as obvious which one would be more effective.
I've had three full-time jobs in different foreign countries, and in all of them, I've walked in and been told to pretend I don't know anything about the local language, and each time I've tried to do so. It has never worked. It's not a question of how well I grade my English, either -- it's simply that you cannot and will not stop the stronger students translating for the weaker ones, which means losing control of the classroom (particularly when the students doing the translation have failed to understand themselves). This translation problem is even more serious in a mixed-mother-tongue class where a large subset of the students already share a common language, where there is a large heterogeneous immigrant population (I'm told there are a lot of South Americans in Italy, for example, and they would have to learn Italian.)
An English-only classroom presents a huge psychological barrier, as the students see themselves as unable to communicate with the teacher. English should be a means of communication, yet we present it to our students in such a way as to make it an impediment to communication. That can't be right, surely....