Nationality is only one of many differences that might change the answer to a question. For example, an American asked "how might someone say they have never had any money" might reasonably be answered
I've never had any money
But depending on where they grew up (e.g. the deep-south) and their cultural vernacular, might also reply:
I've never had no money.
Similarly, in Cockney British English, one might reasonably say
I didn't see nuffink guvnar
Which is grammatically incorrect compared with the Queen's English:
I didn't see anything.
Being British does not automatically make you an expert at Cockney English or Queen's English. Indeed - most people in England are pretty bad at Standard English, particularly when it comes to the spoken word.
One other thing to bear in mind is that a number of ELL users may very well have expertise in more than one area of colloquial English. For example, in my case I'm a British National, but spend roughly two thirds of the year in the US.
Should I be only allowed to comment on British English because that's how I grew up, or should I only be allowed to comment on American English, because that's where I spend most of my time? I was born in Scotland, so should that be my flag, even though I'm certainly no expert on the local dialect?
A flag provides little meaningful benefit to ELL if you can only have one - since it doesn't clarify whether the person is an expert in any particular "field" of English.
If you're allowed multiple flags, it just degenerates quickly into "badges", and it becomes difficult to classify at what point someone should "qualify" for the badge (either whether the badge is awarded, or merely assigned by the user).
For instance, should I get a "South African" flag-badge just for having watched a South African movie? Probably not. What about if I visited it on holiday once? Still no? Ok, how about it was a 3 month holiday. How about I live there now, but have only lived there for less than three months? How about six months?
In short, flags are the wrong way of measuring whether someone knows something about any particular English variant. The best way is to keep the status quo, where users post what they think is correct, and when someone else says "That's not how we do it here", this information appears via a comment. That way, the OP or indeed anyone else coming across the post looking for an answer, automatically gets everything that they want: The answer in a range of dialects from which they can choose the most appropriate answer to their needs.