I had posted this earlier but am posting it again. You need to modify the letter based on your education and proficiency.
This is what I wrote and got my English language requirement waived by the visa office:
Proficiency in English Language – Letter of explanation
Dear Immigration Officer:
I pursued my PhD studies at the University of XXXXXXX USA from year 2000 to 2004 and received a PhD degree in XXXXXXX (subject) in XXXXX (year). I successfully completed the English proficiency requirements which were a necessary prerequisite for joining the PhD program by securing 623 points in the Test for English as a Foreign Language (TOEFL) and 600 points on the Verbal section of the Graduate Record Examination (GRE). These requirements have been mentioned in my PhD transcript which has been attached with the application.
As a part of my current job requirement, I am engaged in training undergraduate students in cell culture and molecular genetics and possess the required fluency in English for conducting these activities. I am also actively engaged in scientific research publication at the highest level in the United States. I have authored over 19 research publications and 30 abstracts during my research career. I am also involved in writing research proposals for securing funding from federal and private grant agencies in the United States. I am a reviewer for 9 scientific research journals and my duties include reviewing scientific manuscripts for suitability for publication in the journals; thus I act as a judge of other peoples scientific research work. Thus from the period of August 2000 to December 2007, I have used English as a medium of instruction for satisfying my academic and job requirements. Please see the employment contract and the letter from the employment agency as evidence for the above.
Additionally, I have completed 12 years of primary and secondary education, 4 years of undergraduate education and 2 years of post-graduate education using English as a language medium. I attended the XXXXXX (name of the school)in XXXXXXX (name of the city), India during my 1st to 12th grade and received instruction in English thorough out this period. English was my first language for my Senior Secondary Certificate as well as my Higher Secondary Certificate examinations. Thus I have been trained in English from my 1st grade till my post-graduate studies. Please see the attached copies of these certificates as evidence.
The official documentation for the requirement of English in my education and job settings should act as proof of my English language proficiency.