Letter of Recommendation from English Teacher for Math Ph.D. program? I am a Japanese undergraduate junior student who is thinking of applying to graduate programs in Mathematics in the U.S. and the biggest problem for me is the Letter of Recommendation. I am planning to ask a teacher of my English class for one of my LOR because my senior who went for a grad school(Engineering) in the U.S. did so.
My intention is that she can prove that I have sufficient English skills to study in the U.S.

However, from what I have seen on the Internet, no one seemed to do so and all of LORs have to be from people in relevant fields. Is getting a LOR from an English teacher for Math Ph.D. program good or bad?
Knowledge of the English language is often a hit-or-miss requirement, i.e., either you have a recognized English qualification (TOEFL, IELTS...) and you can study there, or you do not have it and you cannot study there. In many institutions, this is checked by administrative personnel, independently of the selection committee.

Having better English knowledge than other applicants who also have the qualification will in most cases not give you any advantage over them. The selection committee (or the prospective PhD professor, depending on the location) will almost exclusively consider your qualifications in your research field when deciding whether to admit you.

I would recommend trying to obtain recommendation letters from people that can say good things about your skills in your field.