Diversity of Languageer the activity
Language is an integral part of classroom management.
It is what we say and how we say it.
For L2 learners L1 will always have interference in the language learning process
Students often go into their native language when learning English and a teacher that allows this frequently to happen, is failing in their duty to learn the target language.
I would not go to India to learn how to cook Thai food! The principle is the same.
Both teachers and students need to stay in the target language.