The product is a result of the processes that created it. The same thing applies in teaching. If the mentor teaches the students incorrect English grammar, pronunciation, and spelling, the students will, most likely, do the same.
Teach the student the correct way to solder and they will solder correctly. Teach them to speak well and they would do the same.
The english language is considered to be essential and relatively easy to learn, yet there are still teachers who fail to do well
All of the engineering books we use and all the instructions for engineering laboratory activity are written in English. We see and read it frequently and use it everyday, yet most (engineering) people do not see the value of learning one of the most important, yet basic, skill in the engineering field.
This problem is also true for professionals. Even licensed professionals (e.g. licensed professionals in the academic/teaching field) do not see the importance of having good english communication skills.