To follow on again...
Re: "But as I have suggested previously, being a native speaker does not mean you are good at speaking or understanding it."
I remember reading a study where they assessed the English language levels of mostly native speaker students on graduation and put the average level at around CEFR B1 - That's intermediate English and by no means proficient. A native speaker with B1 competence can barely write short essays. I'm not exactly sure how universities can justify graduating them.
Overseas students are usually required to have certifications in English, e.g. IELTS, TOEFL, or Cambridge CAE or CPE, at much higher levels, i.e. B2 - C1, which represents a full 1 or 2 years (6 or more hours per week) of sustained, guided English language study, so the non-native speakers may be skewing the overall English proficiency levels in the upward direction.
In terms of cultural competence and how well people (native or non-native) can make sense of each others' utterances and writings, an interesting concept is that of symbolic interactionism. It really underlines the subjective nature of how we use language to make meaning: https://en.wikipedia.org/wiki/Symbolic_interactionism In short, just how much of what you say and write do other people understand in the ways you intended? Personally, I find research methods like Grounded Theory with its line-by-line coding incredibly difficult to do because I'm interpreting what the people said through my own perspective which inevitably makes the interpreted meanings partly mine. Is that what they really meant to say?!
Monolingualism (speaking only one language) seems to be a mostly European and English speaking countries' (i.e. colonial) phenomenon. In most other countries, it's common for people to speak two or more languages proficiently. It's almost a given among speakers of two or more languages that you have to check what others have understood of what you've said, i.e. to negotiate meaning during a conversation. I've seen native English speakers get unreasonably weary and roll their eyes when multilinguals repeat stuff back to them (i.e. reflecting their understanding back to the speaker) and say essentially the same thing in different ways (rewording, paraphrasing) in order to clarify what they mean. I don't think they really understand what is happening or why. I think monolinguals have a lot to learn from multilinguals