Using the mother tongue of the students can be helpful, especially when giving instructions or to promote comprehension on something difficult like grammar. But there are a lot of techniques that allow you to use solely English in the classroom. Demonstration, examples, and simple activities that build on one another are only a few ideas. You may want to consider the following, especially in the EFL classroom:
1. The classroom may be the only opportunity for students to practice English communicatively.
2. English only in the classroom helps students build problem-solving/intuitive skills. They learn how to listen for key words, link gestures to the words, and follow instructions. When it comes time to travel abroad, they are better equipped for success.
3. Unless the teacher is very, very fluent, he may translate incorrectly, miss nuance, or even get something entirely wrong.
4. It can be discouraging to the students. This is especially true if the teacher gives a translation whenever someone doesn't understand.
5. It can be frustrating. This is especially true if the teacher gives a translation as the students are working out the meaning on their own.
I rarely use Japanese in my classrooms, and rarely need to. However, it does take a bit of work to effectively and clearly explain ideas, set up activities, etc.
I hope these ideas help.