> This deserves a separate discussion. I doubt many English speakers (let alone those who don't live in a particular anglophone country) care to distinguish between English dialects. To us presence of a huge number of these (don't forget en-AU, en-TT, en-ZW etc - there are more!) in the options lists brings only annoyance, especially when one chooses some non-US one and this opens another can of worms.
I definitely do. The biggest difference, as everyone else, has pointed out is the US vs UK spellings.
Realistically, though, beyond that country is a poor indicator for everything else. I want to use DD/MM/YYYY date format in English, but DD.MM.YYYY date format in German. I want to use $1,000 in English, but 1.000 $ in German. This isn't dependent on the country I live in, this is dependant on a combination of a country and language - that could be the country I'm living in, or the country I grew up in (mostly US date format vs not), and it's either the language I'm actively typing in, or the language of the document I'm reading, or the language I'm thinking in (but a computer can't exactly handle that).
Trying to guess the correct combination is tricky, especially if a document is in two languages (e.g., a quotation), and users are lazy and won't switch their IME unless they have to.
What this means in slightly more practical terms is that setting a single "locale" for my device doesn't make sense, but rather I should be able to choose a locale per language (or possibly spelling preferences by language and formatting options by language as separate choices). I'd then pick a language to use the device in, and it would use that languages locale, and tell apps that this language is the preferred language. If an app doesn't provide my preferred language, pull the preferred locale from settings for a language it does support, otherwise use the default set by developers. For some apps, it's a bit more complex, particularly if I'm creating content. GMail or Office would be two good examples, where the UI language might be in English, but the emails or documents are in German, or a combination of German and English.
Even then, I'm sure there are people who need something even more flexible than that.
At the moment, if I set my language to English but my Country to Germany on my iPhone, for example, things occasionally get confused. My UK banking app, for example, pulled the decimal separator from my locale settings for a while and then refused to work because "£9,79" (or whatever it was) isn't a valid amount of money, and I couldn't see a way to fix that without switching my Country in the phone settings. I imagine they fixed it by ignoring my configured locale and always using en-GB, thus defeating the whole point of a locale in the first place.
So yeah, these days it's fairly common to not have a single "locale" that you work in - it's quite possible to want to use two or more but nothing is really set up to handle that well.