I recently read Fritz Leiber's short story "A Pail of Air" (available to read online at Project Gutenberg), after finding the story via this ID question. It's set in a post-apocalyptic world, with a family who believe they are the only survivors after the Earth left the Sun and its atmosphere froze solid. The setting is in what used to be a city, but can we glean, from any context or details in the story, any more information than that? A country or even a continent? What real-world places are mentioned in the story, and is there any pattern among them?
Specific places mentioned are the nuclear facilities at Los Alamos (New Mexico) Brookhaven (New York), Argonne (Illinois) and "around the world" we find that there are colonies in Harwell (near Oxford, UK) and Tanna[sic] Tavu (a so-called "Atom City" in Russia). There's also mention of "a place called China".
I think we can be reasonably certain that the family is American from contextual clues.
The visitors from Los Alamos are likely speaking in English. The family doesn't have any problems speaking to them, which suggests that their in-universe dialogue is also in English (as opposed to being translated for our benefit).
The family use the terms "Ma", "Pa" and "Sis" which are colloquial terms found primarily in British and American English
The father mentions a "pussy cafe". The Poussé Café is a layered cocktail drink appropriated from France but popularised in America.
The father's name is Harry. An unusual name for France, but common for America.
The streets are laid out in blocks ("We are on a hill and the shimmery plain drops away from us and then flattens out, cut up into neat squares by the troughs that used to be streets"). This would be highly unusual for Britain but extremely common for America.
The author is American.
American actors (with American accents) were chosen to portray the family in the 1956 radio adaptation.