"To what extent should I use the students' native language in class?"
(By the way, thats a great phrase for teaching students how to gain thinking time!)
Its a different question for native speakers of English and non-native teachers of English. It will always be highly artificial for the non-native speaker to avoid the shared mother tongue totally. Students of all ages find it odder to speak English to a fellow-national than to a foreigner. You can see this starting with small kids. My nieces started school in French, though both parents were native speakers of English. It was weird that they could chatter to their classmates in French, but would not accept French from family members (like me) who they knew to be English speakers. People with bilingual children report this same compartmentalisation. Ive seen teachers who manage to keep their classes entirely in English, even though theyre non-natives teaching their own fellow-nationals, and I hugely admire their resolve and effort. A teacher in Spain last year explained how she got around the problem of teaching primary children and keeping the lesson in English. Though she was a native speaker, the kids heard her speak to their parents and other teachers in Spanish and at the earliest levels she had to use some Spanish with them. She introduced a glove puppet into class, and the kids accepted that the puppet had to be addressed in English at all times. She could also have conversations with the puppet.
Many teachers dont have a lot of choice due to lack of ability in the students language, including the great majority of new native-speaker arrivals in Japan. Many ELT teachers go to particular countries because they speak the language, so youd find few teachers in France or Mexico who didnt have the choice. I started teaching in multi-lingual situations, where the sole use of English was a huge advantage, because it was natural and there was a reason for it in the situation. Ive found Japan more similar in some aspects to multi-lingual situations because far fewer native speakers have sufficient proficiency to use the students mother tongue effectively.
What happens eventually is that the teacher develops greater proficiency in the students language. There may be a process here, which also happens as the teachers knowledge of grammar grows. The more you know (in their mother tongue/ in grammar) the more youre tempted to use. Teachers go through phases of using the mother tongue more (or explaining grammar more), then very often make a conscious resolve to get back to the earlier stage where they used English more (or let students deduce grammar more).
At the earliest point, native speaker teachers begin by using odd words to create rapport. Even though the lesson is in English, theyll often greet, thank or bid farewell in the students language. Theres a lot of work before they can really explain in the mother tongue, but odd word translation might come in. Where most students have deduced a word in context, and a couple havent, a quick translation is a lot easier than resorting to board drawing, mime, monolingual dictionaries, paraphrase, definition or whatever. Almost certainly you wont beat the fellow students whisper, which might be how you get to learn some new words. Theres a major gap between this and knowing whether the translation was right!
When I was teaching all Arab groups of total beginners in the 70s, I learnt a few teacher survival phrases (along the lines of Sit Down / Be quiet ), and those teaching teenagers often aquire discipline words fast. The funny thing was that Arab students never remarked on these little forays into their own language. Bernard Hartley, whod taught in Saudi Arabia, used to tell me that as Arabic is considered to be the language of God, they just assumed it was divine intervention giving them simultaneous translation. I shared an office for two years with the late John Curtin, as well as writing a book (Survival English) with him. John could explain points in a multi-lingual situation in five or six languages, as well as correcting students use of their own mother tongue. Whenever there was a general election in the UK, the school would hold a mock election for students with teachers role-playing candidates for the parties. John won handsomely by giving his speech in Spanish, Portuguese, French and Italian, with a few phrases in German, Arabic and Hebrew. The other candidates protested at his tactics on the grounds that (a) it was a crowd-pleasing tactic (b) they hadnt understood enough of his speech to respond to his points. I never had that facility or that degree of choice in my own teaching.
One positive use of the mother tongue was with groups who failed to see the purposes of pronunciation, stress and intonation work. I taught several French Canadian groups in the mid 70s. Due to the political and cultural climate at that time they had chosen to learn English in England rather than by going to Canada (their phrase). I found them resistant to pronunciation work, as they didnt want to sound English. I used to get them to work on my French, and persuaded them to correct my pronunciation. This caused great hilarity for them, and removed a lot of the tension of imitating English sounds and patterns. They saw me mess it up, and knew that I would not be appalled if they made errors. We had a lot of fun, and I found myself researching French words and phrases which are easily confused. Seeing me get into trouble in French made them a lot more careful in English.
Often the content of a lesson might prove so inspiring that students will fall into their own language in the discussion phase. This is inevitable in any monolingual situation where paired and group work is used extensively. You cant legislate against it. One teacher I worked with used to impose tiny fines, and get the class to administer them. In the end, with adults, the only way is persuasion. You have to explain why you think they should stay in English as far as possible.
Recently, there has been less effort to stay entirely in English because more complex activities need explanation. The choice is sometimes between a fairly mundane (but easily explained exercise) and a far more complex and perhaps interesting one that might require the use of the mother tongue for setting it up. Its always going to be a matter of choice. By the way, you can usually keep the explanation in English if you apply enough effort.
Non-native speakers have the advantage of high proficiency in both languages. They know the process, the learning order and the common errors better than native speakers because theyve experienced it. When I started teaching full-time, my boss insisted that one should learn to teach by progressing from beginners upwards. Too often, teachers work the opposite way, and do not get to appreciate how the progression operates. Non-native speakers will and should use their advantages by using the mother tongue when its effective to do so.