You may not believe this, but in Japan it is hotly debated whether to teach English in English or in Japanese, or in both. When it comes to teaching English conversation, teaching in English would be the norm (partly because native speakers of English do not usually speak Japanese). However, this can be a problem because 1) Students will find it too hard to understand English grammar if the grammar class is conducted in English. 2) Some teachers cannot speak English well enough to explain what’s written in the textbooks. 3) There is already a common language between the teacher and the students (Japanese), so it feels a bit strange to use another language to give a detailed instruction. 4) Conventional ways of teaching English include translation, which by nature requires Japanese. and 5) Abstract ideas (e.g. utilitarianism) are hard to teach because there are no visual aids available.
Probably more reasons will arise as to why teaching English in English should be avoided or encouraged.
I teach TOEFL, so my opinion applies only to a limited extent (one critically important distinction being that my students are already motivated and able to understand English – in English). I sometimes teach in English and use no Japanese in class, but every time I do this, I find the same phenomenon: Students are more attentive to what is being said.
This is in part because even those students who have lived in English-speaking countries need to concentrate when they hear English, in part because it makes them feel comfortable to be exposed to spoken natural English, and in part because it is simply a fun experience to see a Japanese citizen speak English fluently.
So what should we do? It depends on what the teachers think of using English in class. Do they speak English just for the purpose of making their students feel comfortable? Do they speak English because it’s fun?
As far as I’m concerned, I speak English because I want my students to learn from me. I am sure that they speak English quite well, but their ability is quite limited. In TOEFL (especially in the Speaking Section) your score will remain the same unless your English has changed. It is my job, therefore, to provide them with as many words, phrases, and idiomatic expressions as possible so that they can absorb them and can use them the next time they take the TOEFL test. Fortunately, they listen to me attentively, so this teaching strategy will be quite effective (although there is no statistical data available).
I once heard that whether you can be a good teacher or not depends on how many examples you can provide for the learners. I kind of believe this. I will leave the rest of the learning process to students. After all, they have their own learning styles.