As you say in your question English is the most common language on the Internet.
Also as far as I could see I can find online courses for learning any language using English i.e) English->Other languages.
Considering the fact that you are asking only about online learning it would be easy to decide that if you know English well you make use of a large number of online courses. But in many cases it is not a necessity.
For Example, I could find online help contents for learning Arabic through Tamil. But if I want to learn Arabic through English I just have large number of options online.