I have been a Polish and English teacher since 1976.
Over the years, I have taught many classes. I introduce a new language mainly by listening and at the beginning all lessons consist of listening, speaking as well as repeating or performing only. I teach grammar simply through situations with no reference to grammar rules at all. My students listen and read a lot to improve their language communication skills. I do not take advantage of the bridge language L1.I often find myself thinking in Polish and English, flying back and forth from the two languages.
It is difficult to manage exchanging thoughts in two languages simultaneously. Anyhow, why would you like to? Generally speaking, I am for blended learning and teaching with no remark to L1.
Thinking in a foreign language from an early stage helps students gain a level of fluency making very few errors in communication.
I believe, the sooner a student learns to think in a foreign language, the faster she/he will learn. This can only take place if no reference at all is made to L1. My learners need to be free from the interruption of L1.
Maybe I am wrong because it is obviously much easier to teach/learn with the help of pure conversion.
On the other hand, I know that anyone who is learning a second language wishes and dreams about communicating and thinking in that language.
I teach a lot, approximately 4 Polish classes and 4 English classes a day, and I have to admit that my students are very successful learners.