Laiza you can include the following points in your letter:
1- Working in the company has an English speaking environment Or English is the main medium in your organization and
2- You use all four components of english language (Such as Listening, Speaking, Writing and Reading) in your communication in your organization.
Suggestion, write as well that you've gone through seminars, training that requires paticipation and english is medium of language. I'm planning to write as wrtten as well instead of submitting ietls result. How was it, have you send yours?
