Letter of Recommendation from English Teacher for Math Ph.D. program?
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.