A teacher needs to know students' native language in order to teach them effectively. If the student and teacher do not speak the same language, they cannot communicate at all. Often, a teacher will have access to the academic records of their students so that they can make effective teaching plans.
A teacher needs to be able to understand the native language of their student. Certain ideas cannot be properly conveyed if their is a language barrier. It is impossible for a teacher to identify if a student does not understand the concept or just does not understand the words that are coming out of their mouth.
Knowledge of a student's native language is important early on in being an effective teacher. In the early years the studdent is going to best learn through their native language. Once they reach a certain age and skill level at other languages though this is no longer the case in teaching the student.
Knowing a student's native language is a way for teachers to break down language barriers. It doesn't have to be a lot of knowledge, but just enough to get a child's attention no matter if the language is Spanish, French or even Farsi. A little forethought goes a long way with regards to teachers making impressions on students.
When you are teaching a kid you will need to be able to connect with them on an emotional way as well as an intellectual way. Knowing the students initial language can be a great stepping stone in order to connect intellectually and emotionally with a student in the classroom.