One problem I've encountered is that in the past year, over 90% of my interviews have been for international applicants that seem to struggle with anything beyond basic English. I end up having to guide them through the whole conversation, so they get marked down for needing the hints, and usually perform worse for having less time.
I've been told by colleagues that my questions are unfair for this reason, and that I should make my questions more inclusive by making the requirements clear, but I feel like not being able to discuss an ambiguous problem in the company's language is a big red flag, regardless of the reason.
And I DO NOT CARE if it is "right" or not!
I want to hear them state assumptions, expectations, etc!
Your relative grasp of English (in my case) is [almost] 100% irrelevant
I want to hear you
Ask me for clarification
Ask me to explain a word/term you do not understand
Then show me how you think!
It doesn't work to hire an accounting genius, whose English isn't up to understanding the IRS instructions, forms, & code, to handle your taxes. But if you (in effect) required a polished Harvard accent for that job, you'd deserve to lose a discrimination lawsuit.
However, if you are deliberately being ambiguous, communicating in a way that is designed to be trickier than a normal conversation, yes, that is completely unfair.