What platform/language to choose when creating an online course?

What are the objectives of your English course? Without knowing the objectives, it is hard to know how to advise you about what authoring environment will be the best fit.

Is this a course about reading? About writing? About speaking? Is it for native speakers of English or for students whose first language is not English? What kind of activities/assignments will the course ask students to complete? Will those be graded/assessed in the course or outside the course by an instructor (e.g., you)? Will students be taking this online course as a component of a blended learning curriculum or as a stand-alone lesson? Will students be taking the course together as a cohort (meaning, you might want them to be able to have discussions with each other) or individually on a self-paced basis? 

With more understanding about your goals it will be easier to recommend possible authoring environments.