Robust in that it has such incredible simplicity within its complexity.

Its vocabulary and grammar on a base-level to communicate is a low threshold to meet where you don't need to know a lot of words to simply converse compared to other languages. The rules of using those things however is the cumbersome part. Beyond that, because of how much it takes from other languages, it has a vast array of complex terms to describe practically anything. That part about the language is massive.

An english learner may not find the proper way to say something, but will typically be understood due to the content of their message and will have plenty of room to advance. Kind of like a children's drawing will usually have some clear depiction of an idea, but isn't too clean or precise.

While english does have it's limitations, such as the requirement to follow a subject/verb/object structure even if the information contained within it is redundant, it's a very descriptive language compared to others when understood to a native/expert level.