* calc/calc.el (calc-gregorian-switch): In menu, put dates before regions.
This is easier to follow, lines up better in the menu, and lets us coalesce regions that switch at the same time. Give country names, not "Vatican", as that's better for non-expert users. Use names that are stable between the date of switch and now, e.g., Bohemia and Moravia (which existed then and now) and not Czechoslovakia (which didn't exist then and doesn't exist now). What is now the U.S. mostly did not switch at the same time as Britain, so omit the U.S. Correct spelling of "Britain". Catholic Switzerland was too much of a mess, so omit it.
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