The United States is a large country, and I've lived all over the US. I know how people talk in the northeast, and how they talk in the south. I can recognize a Carolinian accent and distinguish it from a Tennesseean accent, and I can hear a single word in your speech that tells me you're from Ohio, or an expression that tells me you're from the Dakotas.
But I can also be wrong about all of that.
I have family members who have never lived outside of Chicago, but who speak as if they were from Kentucky. I have friends who use inherently "British" words in spite of having never left the United States. And I have seen television and popular culture and low-cost airfare make the world a lot smaller for everyone.
Hey, that's regional
A I learn Italian, I've encountered plenty of words and phrases (especially slang) which I am told are specific to a particular city or region, even in spite of the plain reality that I've learned them from people who do not live in or come from those regions. In fact, I've had several people tell me that "many of those expressions" are only used in Rome, even as I learn them from people in Milan.
Of course I experienced the same thing last year, as I had Russians tell me I was using Ukrainian slang, while the Ukrainians told me it was Georgian, and the Uzbekis said it was Siberian. One person says "they only say that in Moscow", and the next person says "I live in Moscow and I've never heard that".
And Spanish was the same way, too. I actually have several books of Spanish slang, which go to great lengths to indicate in which countries and regions a particular expression is used. Yet I hear these things used by people not listed on those regions, while people who are from there don't have a clue what I'm talking about.
The effect of capital cities
When people say "they only say that in Rome/Moscow/Madrid/Mexico City/etc" there's a reason for that: those capital cities are where everything happens. The majority of news comes from the capital, and the majority of film and television comes from the biggest city, which is also usually the capital.
When people come from around the country, or even around the world, to visit Russia, they're not going to Irkutsk. And when they come from around the country or the world to visit Italy, the first place on their minds isn't Puglia, it's Rome. If you're going to France, you're not thinking Montpelier, you're thinking Paris.
All the regional slang from everywhere ends up in these main cities. And more importantly, with so much going on there, most of the new slang originates there. But eventually, it makes its way back to other parts, whether by broadcast or just from people returning home and bringing it with them.
But who cares?
In my opinion, when it comes to how regional a particular phrase is, the bottom line is who cares? If I'm talking to someone from England and they use an obviously English expression, I still don't have any problem understanding them. And more importantly, the fact that they're from England holds no importance over what they've said.
I have made several friends around the world who practice their English with me even as I practice my Russian or Italian or Spanish with them, and I often find that while Spanish and Italian people seem to learn an American style of English, Russians tend to learn a decidedly more British form. If I were to stop them after every sentence and say "you sound British", or "we don't say that here", the conversation would go nowhere. Instead, we just talk. Who cares?
And most importantly, if I'm learning a language, I want to understand that language, regardless of where I am. If I were to avoid learning regional words, I would be great when reading official documents, but I would risk being totally lost when standing there in the country, talking to an actual person.
Sure, if I were to move to Italy, I might pay more attention to learning words that were used in the city where I lived — but that wouldn't be so strange since I'd be surrounded by that regional vocabulary every day. And perhaps if I were to then move to another region, I might become much more interested in the "regionality" of certain expressions.
But I'm an American, living in Chicago, learning to speak Italian in one year. And I expect that most people reading this blog are similar — English-speaking people, living in an English-speaking country, learning a foreign language. So I say don't worry about whether or not a phrase is regional. Who really cares if this phrase is used in this city, or that phrase in that one, when they're all 100% more Italian/German/Russian/French/Thai/whatever than any of the 825 words you just read?